Fix: Handle invalid GenBank accessions or missing RefSeq accessions in _resolve_genbank_accession()

[liannette]

Fixes issue #307 + additional issue

This PR addresses two different bugs in the _resolve_genbank_accession() function when resolving GenBank accession numbers to RefSeq accessions:

1. The function previously failed when a valid GenBank accession existed in NCBI but had no corresponding RefSeq accession.
2. It also broke when the provided GenBank accession was invalid (not found in NCBI).

Changes & Improvements:

• Instead of causing an error, the function now returns an empty string ("") when no RefSeq accession is available.
• Additional error handling cases have been added to improve robustness.
• The function has been refactored for better readability and maintainability.