fix(spannerlib): gracefully handle missing stats for single statements

[surbhigarg92]

In readStats, normal (non-profiled) single-statement queries do not
return a trailing stats result set. The previous implementation returned
a "stats results not found" error, which disrupted normal execution.
This fix returns nil instead, allowing the EOF auto-close logic to
complete successfully without phantom errors.

[gemini-code-assist]

Code Review

This pull request refactors the nextBatch method in spannerlib/api/rows.go to unconditionally read statistics, and modifies readStats to return nil instead of an internal error when there are no more result sets. The feedback suggests returning rows.backend.Err() instead of nil unconditionally when NextResultSet() returns false, in order to avoid swallowing underlying errors.

[gemini-code-assist]

When rows.backend.NextResultSet() returns false, it can be either because there are no more result sets or because an error occurred during the operation. Returning nil unconditionally will swallow any such errors. Instead, return rows.backend.Err() to correctly propagate any underlying errors while still returning nil when there are no more result sets.

Suggested change

	if !rows.backend.NextResultSet() {
	return status.Error(codes.Internal, "stats results not found")
	return nil
	}
	if !rows.backend.NextResultSet() {
	return rows.backend.Err()
	}