getNewFieldWriter returns UnionListWriter instead of UnionMapWriter

[wsuppiger]

Not sure if I am using this wrong, but I noticed getNewFieldWriter for the minorType MAP is returning a UnionListWriter instead of a UnionMapWriter.

arrow-java/vector/src/main/java/org/apache/arrow/vector/types/Types.java

Lines 722 to 726 in 799d9fd

	@Override
	public FieldWriter getNewFieldWriter(ValueVector vector) {
	return new UnionListWriter((MapVector) vector);
	}
	},

This leads to the issue of this code block throwing an error:

writer = MinorType.MAP.getNewFieldWriter(myMapVector)
writer.startMap()

Exception:

You tried to start a map key when you are using a ValueWriter of type UnionListWriter.
java.lang.IllegalStateException: You tried to start a map key when you are using a ValueWriter of type UnionListWriter.
	at org.apache.arrow.vector.complex.impl.AbstractFieldWriter.key(AbstractFieldWriter.java:123)
	at org.apache.arrow.vector.complex.impl.UnionListWriter.key(UnionListWriter.java:69)
...

I believe this code can be easily fixed by just changing it to:

   @Override 
   public FieldWriter getNewFieldWriter(ValueVector vector) { 
     return new UnionMapWriter((MapVector) vector); 
   } 

[lidavidm]

PRs would be appreciated!