chore: use a single shared NullValue instance

[olavloite]

Follow-up for #720

[gemini-code-assist]

Summary of Changes

Hello @olavloite, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a micro-optimization by refactoring how null values are handled during parameter conversion. By creating and reusing a single, shared instance of structpb.NullValue, the change aims to reduce redundant object allocations and potentially improve memory efficiency within the go-sql-spanner driver.

Highlights

• Shared NullValue Instance: A package-level variable nullValue has been introduced in stmt.go to hold a single, shared instance of structpb.NullValue.
• Optimization in Parameter Conversion: The convertParam function in stmt.go has been updated to reuse this nullValue instance instead of creating a new structpb.NewNullValue() object each time a null parameter is encountered, reducing object allocation.

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[gemini-code-assist]

Code Review

This pull request introduces a good optimization by using a single shared instance for structpb.NullValue. This is achieved by creating a package-level variable nullValue and using it to replace repeated calls to structpb.NewNullValue() within the convertParam function. This change effectively reduces memory allocations in a potentially hot path, improving performance. The implementation is correct, simple, and follows best practices for this kind of optimization. I have no further comments.

[bhatt4982]

LGTM