Write struct support

[stevelordbq]

No description provided.

[stevelordbq]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for writing Spark Struct and Array[Struct] types to Google Cloud Spanner. A critical flaw was identified in the handling of struct arrays, where the code incorrectly assumes the first element is non-null when determining the element type. This can lead to a NullPointerException and a potential Denial of Service if the input data contains a null as the first element of such an array. Beyond this, there are critical issues regarding the general handling of null and empty values for structs and struct arrays, which could lead to data correctness problems or runtime exceptions. The tests also contain some hardcoded values and incorrect assertions that should be addressed.

[gemini-code-assist]

This test asserts that a null struct should be converted to an empty struct. This is inconsistent with how other null types are handled and validates incorrect behavior. A null struct from Spark should result in a NULL value in Spanner.