[SPARK-9831] [SQL] fix serialization with empty broadcast

Author: Davies Liu <davies@databricks.com>

Closes #8117 from davies/fix_serialization and squashes the following commits:

d21ac71 [Davies Liu] fix serialization with empty broadcast

sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashedRelation.scala

@@ -299,7 +299,7 @@ private[joins] final class UnsafeHashedRelation(
     binaryMap = new BytesToBytesMap(
       taskMemoryManager,
       shuffleMemoryManager,
-      nKeys * 2, // reduce hash collision
+      (nKeys * 1.5 + 1).toInt, // reduce hash collision
       pageSizeBytes)
 
     var i = 0

sql/core/src/test/scala/org/apache/spark/sql/execution/joins/HashedRelationSuite.scala

@@ -103,4 +103,21 @@ class HashedRelationSuite extends SparkFunSuite {
     assert(hashed2.get(unsafeData(2)) === data2)
     assert(numDataRows.value.value === data.length)
   }
+
+  test("test serialization empty hash map") {
+    val os = new ByteArrayOutputStream()
+    val out = new ObjectOutputStream(os)
+    val hashed = new UnsafeHashedRelation(
+      new java.util.HashMap[UnsafeRow, CompactBuffer[UnsafeRow]])
+    hashed.writeExternal(out)
+    out.flush()
+    val in = new ObjectInputStream(new ByteArrayInputStream(os.toByteArray))
+    val hashed2 = new UnsafeHashedRelation()
+    hashed2.readExternal(in)
+
+    val schema = StructType(StructField("a", IntegerType, true) :: Nil)
+    val toUnsafe = UnsafeProjection.create(schema)
+    val row = toUnsafe(InternalRow(0))
+    assert(hashed2.get(row) === null)
+  }
 }