Improve robustness by safely accessing metadata using dict.get

[tejasnarwade]

This PR improves robustness in RPWSpectrogram by replacing direct dictionary access (meta["instrument"]) with meta.get("instrument").

This prevents potential KeyError exceptions when the "instrument" key is missing, making the code more resilient to incomplete metadata.
This was small change but still i think prevention is better than cure so i pushed the pr
The behavior remains unchanged when the key is present.

PR Description
AI Assistance Disclosure
AI tools were used for:

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding
• No AI tools were used**

[samaloney]

Thanks for the PR but I'm not sure we want to do this, if the metadata isn't there it indicates problem with the source file and these changes would hide that.

Also do you have an example of this happening in the real world? I think I'd like to see a failing test case on real data before proceeding down this route.