Parquet: Override `variant()` in `ParquetAvroWriter.WriteBuilder` to prevent null writer/NPE

[manuzhang]

Feature Request / Improvement

ParquetAvroWriter.WriteBuilder does not override variant(...), so it inherits the default implementation from ParquetTypeVisitor, which returns null.

During struct writer construction, that null field writer is wrapped with ParquetValueWriters.option(...) for OPTIONAL fields, and OptionWriter dereferences writer.columns(), causing a NullPointerException during writer construction.

Although being only used in tests, ParquetAvroWriter is still a public API, so it would be nice to fix the NPE.

Query engine

None

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time

[nssalian]

@manuzhang, I'm happy to work on this. Do you mind assigning it to me - I can put up a PR for it.