### What changes were proposed in this pull request?
When `peakMemoryMetrics` in `ExecutorSummary` is `Option.empty`, then the `ExecutorMetricsJsonSerializer#serialize` method does not execute the `jsonGenerator.writeObject` method. This causes the json to be generated with `peakMemoryMetrics` key added to the serialized string, but no corresponding value.
This causes an error to be thrown when it is the next key `attributes` turn to be added to the json:
`com.fasterxml.jackson.core.JsonGenerationException: Can not write a field name, expecting a value
`
### Why are the changes needed?
At the start of the Spark job, if `peakMemoryMetrics` is `Option.empty`, then it causes
a `com.fasterxml.jackson.core.JsonGenerationException` to be thrown when we navigate to the Executors tab in Spark UI.
Complete stacktrace:
> com.fasterxml.jackson.core.JsonGenerationException: Can not write a field name, expecting a value
> at com.fasterxml.jackson.core.JsonGenerator._reportError(JsonGenerator.java:2080)
> at com.fasterxml.jackson.core.json.WriterBasedJsonGenerator.writeFieldName(WriterBasedJsonGenerator.java:161)
> at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:725)
> at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:721)
> at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:166)
> at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145)
> at com.fasterxml.jackson.module.scala.ser.IterableSerializer.serializeContents(IterableSerializerModule.scala:26)
> at com.fasterxml.jackson.module.scala.ser.IterableSerializer.serializeContents$(IterableSerializerModule.scala:25)
> at com.fasterxml.jackson.module.scala.ser.UnresolvedIterableSerializer.serializeContents(IterableSerializerModule.scala:54)
> at com.fasterxml.jackson.module.scala.ser.UnresolvedIterableSerializer.serializeContents(IterableSerializerModule.scala:54)
> at com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:250)
> at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
> at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
> at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:4094)
> at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3404)
> at org.apache.spark.ui.exec.ExecutorsPage.allExecutorsDataScript$1(ExecutorsTab.scala:64)
> at org.apache.spark.ui.exec.ExecutorsPage.render(ExecutorsTab.scala:76)
> at org.apache.spark.ui.WebUI.$anonfun$attachPage$1(WebUI.scala:89)
> at org.apache.spark.ui.JettyUtils$$anon$1.doGet(JettyUtils.scala:80)
> at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
> at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
> at org.sparkproject.jetty.servlet.ServletHolder.handle(ServletHolder.java:873)
> at org.sparkproject.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1623)
> at org.apache.spark.ui.HttpSecurityFilter.doFilter(HttpSecurityFilter.scala:95)
> at org.sparkproject.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
> at org.sparkproject.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:540)
> at org.sparkproject.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:255)
> at org.sparkproject.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1345)
> at org.sparkproject.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:203)
> at org.sparkproject.jetty.servlet.ServletHandler.doScope(ServletHandler.java:480)
> at org.sparkproject.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:201)
> at org.sparkproject.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1247)
> at org.sparkproject.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:144)
> at org.sparkproject.jetty.server.handler.gzip.GzipHandler.handle(GzipHandler.java:753)
> at org.sparkproject.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:220)
> at org.sparkproject.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
> at org.sparkproject.jetty.server.Server.handle(Server.java:505)
> at org.sparkproject.jetty.server.HttpChannel.handle(HttpChannel.java:370)
> at org.sparkproject.jetty.server.HttpConnection.onFillable(HttpConnection.java:267)
> at org.sparkproject.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:305)
> at org.sparkproject.jetty.io.FillInterest.fillable(FillInterest.java:103)
> at org.sparkproject.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:117)
> at org.sparkproject.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:333)
> at org.sparkproject.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:310)
> at org.sparkproject.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:168)
> at org.sparkproject.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:126)
> at org.sparkproject.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:366)
> at org.sparkproject.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:698)
> at org.sparkproject.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:804)
> at java.base/java.lang.Thread.run(Thread.java:834)
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Unit test
Closes#29872 from shrutig/SPARK-32996.
Authored-by: Shruti Gumma <shruti_gumma@apple.com>
Signed-off-by: Liang-Chi Hsieh <viirya@gmail.com>
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