[Java] nullable arrow schema write to parquet error

### Describe the usage question you have. Please include as many useful details as possible.

When I convert nullable avro schema to nullable arrow schema and write as parquet file， meet a exception

"Unhandled type for Arrow to Parquet schema conversion: sparse_union"

It seems arrow-avro not support SPARE_UNION

how can I use nullable schema in arrow, please help me.

### Component(s)

Java

Start by reproducing the reported conversion from a nullable Avro schema to a nullable Arrow schema, then write it as a Parquet file and capture the sparse_union failure. Confirm whether the conversion path supports this schema and document the expected behavior or limitation; no source files or tests are identified in the issue.

Written by the indexing model from the issue text.