Ensure that the transcripts variable is always initialized before use to prevent UnboundLocalError:

Traceback (most recent call last):
  File "/home/training.imgge/add_most_severe_pli.py", line 129, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/training.imgge/add_most_severe_pli.py", line 125, in main
    write_pli_annotated_vcf(in_vcf, out_vcf)
  File "/home/training.imgge/add_most_severe_pli.py", line 90, in write_pli_annotated_vcf
    vcf_record = construct_most_severe_pli_info(line, pli_ind)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/training.imgge/add_most_severe_pli.py", line 53, in construct_most_severe_pli_info
    pli_values = parse_vep_transcripts(transcripts, pli_ind)
                                       ^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'transcripts' where it is not associated with a value

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• If necessary, also make a PR on the nf-core/raredisease branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Ensure the test suite passes (nextflow run . -profile test_singleton,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).