[Avro] Accept dict with only `'type': 'null'` as representation of `null`#2109

Tishj wants to merge 3 commits intoapache:mainapache/iceberg-python:mainfrom

Tishj:support_different_avro_null_representationTishj/iceberg-python:support_different_avro_null_representationCopy head branch name to clipboard