fix(bigquery): support optional fields in BigLakeConfiguration to prevent NPE on Iceberg/Lakehouse tables

[keshavdandeva]

b/534291682

This PR resolves a NullPointerException encountered when querying or retrieving metadata for 4-part PCNT (Project.Catalog.Namespace.Table) Apache Iceberg tables governed by external REST catalogs.

Changes

• Annotated all 4 fields (connectionId, fileFormat, storageUri, tableFormat) and their corresponding setters in @AutoValue.Builder with @Nullable in BigLakeConfiguration.java.
• Cleaned up the Javadoc comments for the builder setters in BigLakeConfiguration.java to remove incorrect [Required] notations and updated getTableFormat() Javadoc to reflect the correct table format reference (e.g., Iceberg) instead of file format.
• Added unit test coverage to BigLakeConfigurationTest.java (testNullFields, testFromPbWithNullFields) to verify that constructing the configuration or mapping it from API models works successfully when properties are null.

[gemini-code-assist]

Code Review

This pull request updates BigLakeConfiguration to allow nullable fields, adding @Nullable annotations to the getters and builder setters, and removing the "[Required]" designation from the builder's Javadocs. It also adds unit tests to verify behavior when fields are null. The review feedback identifies copy-paste errors in the Javadocs for getTableFormat() and setTableFormat(), which incorrectly refer to "file format" and "PARQUET" instead of "table format" and "ICEBERG".

[keshavdandeva]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates BigLakeConfiguration by adding @Nullable annotations to its getters and builder setters, updating Javadoc comments, and adding unit tests to verify null field handling. The feedback suggests using the more idiomatic assertNull(value) instead of assertEquals(null, value) in the new test cases.