chore(spanner): reuse CharsetEncoder in ChecksumResultSet

[olavloite]

Reuse the CharsetEncoder in ChecksumResultSet to prevent the creation of a new encoder for each string that we encounter.

[gemini-code-assist]

Code Review

This pull request optimizes string encoding in ChecksumResultSet by reusing a CharsetEncoder instance in ChecksumCalculator instead of instantiating a new one on every putString call. The reviewer suggested eagerly initializing the encoder as a final field and directly calling encoder.reset() to simplify the code and avoid unnecessary null checks.

[gemini-code-assist]

Instead of lazily initializing the encoder field with a null check in putString, we can declare it as final and initialize it directly at the declaration site. This simplifies the code and avoids unnecessary branching during execution.

Suggested change

	private CharsetEncoder encoder;
	private final CharsetEncoder encoder = StandardCharsets.UTF_8.newEncoder();

[gemini-code-assist]

Since the encoder is now eagerly initialized and declared as final, we can replace the null check with a direct call to encoder.reset().

      encoder.reset();