Lost Nullability in Calcite Roundtrip

[gord02]

When a nullable field type is defined in the schema of a virtualTableScan, the fields are enforced to also be nullable. Calcite's logicalValues does not enforce this match of the schema and actual field implementation. The following failing test case exposes this bug.

@Test
  void lostNullabilityInVirtualTableOnRoundTrip() {
      NamedStruct schema = NamedStruct.of(List.of("col1", "col2"), R.struct(N.I32, R.FP64));
      Expression nullableI32 = Expression.I32Literal.builder().value(6).nullable(true).build();
      VirtualTableScan virtualTableScan = createVirtualTableScan(schema, List.of(nullableI32, sb.fp64(8)));
      assertFullRoundTrip(virtualTableScan);
  }

A workaround should either be implemented in substrait-java(potentially loosening the schema matching), or a fix in Calcite should be implemented(logicalValues should update the field list type definition when making its fields non-null).

[asolimando]

The root cause is in SubstraitRelVisitor.visit(Values): when converting Calcite LogicalValues back to Substrait, the code was using the RexLiteral type's nullability instead of the schema's nullability.

Calcite's LogicalValues doesn't enforce that literal types match the declared row type, so a non-null value like 6 may have a non-nullable type even when the schema field is nullable (Calcite infers the tightest possible type for each literal, that's by design).

The fix uses the schema field nullability when converting each literal back to Substrait:

• Added LiteralConverter.convert(RexLiteral, boolean nullable) overload that accepts target nullability explicitly
• Modified SubstraitRelVisitor.visit(Values) to pass the schema field nullability when converting literals

This approach preserves the strict validation in VirtualTableScan while handling Calcite's loose type enforcement during roundtrip.

I have a PR ready that I will push shortly (cc: @benbellick @gord02)