Coral-Schema: Return nullable schema for inner fields created by operator

[ljfgem]

Change Summary

At present, we are using RelDataType.isNullable() to determine if an inner field should be nullable or not while converting RelDataType to Avro schema, but Spark expects all the inner fields created by operator to be nullable. This PR is to make Coral-Schema return nullable schema for inner fields created by operator to be aligned with Spark's expectation.

Tests

1. Modified UTs, these changed fields are created by operator, they are nullable now with this PR
2. Regression test on all the prod views, no translation regression
3. Tested on the affected prod views, can be queried on Spark shell with this PR

[yiqiangin]

LGTM. It looks like this change is done for Spark. Pls make sure it does work for other query engines as well.

[ljfgem]

LGTM. It looks like this change is done for Spark. Pls make sure it does work for other query engines as well.

Thanks for review! Coral-Schema is only used for view queries on Spark at present.