Treat blank optional string arguments as absent

[mysticfall]

Make getOptionalString return null for blank strings so that optional parameters with empty/whitespace-only values are treated the same as unset parameters.

This fixes an issue where invoking tools such as search_security_hotspots on a local project without a pull request ID fails, because the SonarQube Server's endpoint rejects a URL with an empty pullRequest parameter.

It's reproducible with OpenCode, using either a Docker or jar version of the project.

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