UnionMapWriter fails to write fixed size binary

[nbauernfeind]

A UnionMapWriter will null out the entire map struct entry instead of setting the value to null:

 childWriter.value().fixedSizeBinary().writeNull();

Actually hits NullableStructWriter#writeNull whereas UnionMapWriter should override writeNull and do a switch on the mode (though I'm not certain if null keys are valid).

EDIT:
Actually, the issue is that fixedSizeBinary needs to be overriden! Very frustrating, but this call is not overridden and is instead invoking UnionListWriter#fixedSizeBinary():

 @Override
 public FixedSizeBinaryWriter fixedSizeBinary() {
 return this;
 }

The real impl needs to override like so:

 @Override
 public FixedSizeBinaryWriter fixedSizeBinary() {
 switch (mode) {
 case KEY:
 return entryWriter.fixedSizeBinary(MapVector.KEY_NAME);
 case VALUE:
 return entryWriter.fixedSizeBinary(MapVector.VALUE_NAME);
 default:
 return this;
 }
 }

[jbonofre]

Let me try to reproduce the issue on a test (and fix it).

[mihazagar]

@jbonofre , I’d like to tackle this issue!

Please let me know if there are any details you’d like me to keep in mind.