### Why are the changes needed?
As mentioned in jira, sometimes we need to be able to support the retention of null columns when writing JSON.
For example, sparkmagic(used widely in jupyter with livy) will generate sql query results based on DataSet.toJSON and parse JSON to pandas DataFrame to display. If there is a null column, it is easy to have some column missing or even the query result is empty. The loss of the null column in the first row, may cause parsing exceptions or loss of entire column data.
### Does this PR introduce any user-facing change?
Example in spark-shell.
scala> spark.sql("select null as a, 1 as b").toJSON.collect.foreach(println)
{"b":1}
scala> spark.sql("set spark.sql.jsonGenerator.struct.ignore.null=false")
res2: org.apache.spark.sql.DataFrame = [key: string, value: string]
scala> spark.sql("select null as a, 1 as b").toJSON.collect.foreach(println)
{"a":null,"b":1}
### How was this patch tested?
Add new test to JacksonGeneratorSuite
Closes#26098 from stczwd/json.
Lead-authored-by: stczwd <qcsd2011@163.com>
Co-authored-by: Jackey Lee <qcsd2011@163.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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