Return 404 instead of 500 for missing reference genome gene

[Gautam-aman]

Problem

Requesting a reference genome gene that does not exist results in a 500 server error.

Solution

Add explicit null handling in ReferenceGenomeGeneController and return HTTP 404 when the gene is missing.

[Gautam-aman]

This PR applies a minimal defensive fix to return HTTP 404 instead of 500
when a reference genome gene is missing.

No business logic was changed and no new dependencies were introduced.
Please let me know if you would like me to add a controller test.