Parquet: Fix NPE in value reader building for projections

[rdblue]

The visitor that builds Parquet value readers for Iceberg generics did not handle cases where the expected type was null because a field was is not projected. This was not a problem for most primitive types that did not use the expected type, but cases that did would throw a NullPointerException.

Fixes #1117.

[rdsr]

I was trying to look into why this was only a failure for Parquet. I might be wrong here, but It seems that for ORC and Avro were are building the readerFuncs in TestReadProjection.writeAndRead using Iceberg expected schema and format specific read schemas, whereas in Parquet we are using Iceberg expected schemas and the format specific file schema. Any thoughts on why that is the case?

[rdsr]

Also should this be addressed for other formats?

[rdsr]

+1

[rdblue]

@rdsr, the test applies to all formats and only Parquet was broken.

[rdblue]

Any thoughts on why that is the case?

The equivalent Avro builder doesn't use the expected primitive at all. The variable name is ignored. But looking at the ORC one, I think there would be a similar problem. I assumed it would have been caught by the test, but I guess we need to fix this there as well.

[rdsr]

I assumed it would have been caught by the test, but I guess we need to fix this there as well.

I believe it is not getting caught because in ORC (as well as Avro) the reader func are utilizing the read schema which completely match the Iceberg expected schema so there will not be a case where the iceberg type is null and the format type is not, whereas in Parquet the reader func is using the file schema.