feat: implement wasNull for BigQueryResultSet

[PhongChuong]

• Implement wasNull as defined by java.sql.ResultSet for BigQueryResultSet.
• Add a new unittest file for BigQueryResultSet
• Add unittest for BigQueryResultSet getters for both FieldValue type buffer and read API type buffer
• Perform minor refactoring in BigQueryResultSet

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #3648 ☕️

[whuffman36]

LGTM

[shollyman]

This code seems fairly invasive with the hasNull plumbing, and I worry it'll be prone to error. Could we move the hasNull state computation higher up into when we do cursor advancement? Then it's just something like (pseudocode) for field in row.fields { if field.isNull() { hasNull = true }}?

[shollyman]

This code seems fairly invasive with the hasNull plumbing, and I worry it'll be prone to error. Could we move the hasNull state computation higher up into when we do cursor advancement? Then it's just something like (pseudocode) for field in row.fields { if field.isNull() { hasNull = true }}?

After reading more about java.sql.resultset, it seems my previous understanding was wrong. It's not whether the previous row had a null, its if the latest field fetched had a null, which does yield more complexity here.

[shollyman]

Marking this LGTM, modulo some comment/docs updates to clarify some of the details here we discussed out of band.

[PhongChuong]

Thanks for the reviews.