Add test for missing instrument metadata in RPWSpectrogram

[tejasnarwade]

This PR adds a test to check the behavior of RPWSpectrogram when the
"instrument" key is missing from the metadata.

Previously, accessing this key directly could raise an error. With the
recent change to use a safer access method, this test ensures that the
function handles such cases gracefully and returns False instead of
failing.

This helps improve reliability and prevents similar issues in the future.

PR Description
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