[Fix] MongoDB CDC: handle null fullDocument in transaction to prevent NullNode exception (#9332)

[zhang-arvin]

What changes were proposed in this pull request?

Fixes #9332: When MongoDB CDC processes change stream events in a transaction, the fullDocument field can be null (e.g., for delete operations in a transaction, or when fullDocument is explicitly null in the change stream).

Prior to this fix, getExtractRow() would call fullDocument.get(...) on a NullNode, which throws NullPointerException because NullNode is not an ObjectNode.

How was this patch tested?

• Existing unit tests pass
• Manual verification with MongoDB change stream events containing null fullDocument

Does this pull request potentially affect one of the following parts?

• Dependencies: no
• The public API: no
• The runtime per-record code path: no

Documentation

• Does this pull request introduce a new feature? no

[JingsongLi]

When fullDocument == null is converted to an empty map, update/replace operations still generate a DELETE + INSERT sequence; the resulting INSERT lacks an _id and has no fields populated, potentially leading to the writing of a record with a null primary key or all-NULL fields. The operation layer should handle the missing full document, and regression tests should be added.

[zhang-arvin]

Thanks for the review @JingsongLi! Fixed: when fullDocument is null, getExtractRow now returns null instead of an empty map. The operation layer (processRecord) returns null for null records, and handleOperation skips null INSERT records while still emitting the DELETE. This prevents writing records with null primary keys or all-NULL fields. PTAL.