Czhu fix summarize fasta

[zhuchcn]

Description

--variant-peptides is made optional now.

Closes #482

Checklist

• [X] This PR does NOT contain PHI or germline genetic data. A repo may need to be deleted if such data is uploaded. Disclosing PHI is a major problem.
• [X] This PR does NOT contain molecular files, compressed files, output files such as images (e.g. .png, .jpeg), .pdf, .RData, .xlsx, .doc, .ppt, or other non-plain-text files. To automatically exclude such files using a .gitignore file, see here for example.
• [X] I have read the code review guidelines and the code review best practice on GitHub check-list.
• [X] The name of the branch is meaningful and well formatted following the standards, using [AD_username (or 5 letters of AD if AD is too long)]-[brief_description_of_branch].
• [X] I have added the major changes included in this pull request to the CHANGELOG.md under the next release version or unreleased, and updated the date.
• [X] All test cases passed locally.

[lydiayliu]

Can you also address the same thing for splitFasta in this PR? https://github.com/uclahs-cds/private-moPepGen/issues/482#issuecomment-1856528933

[lydiayliu]

@zhuchcn sorry what is the status on this XD (not that I need it now but just saw this PR again lol

[zhuchcn]

Just fixed that!

[zhuchcn]

That's correct. Just like the test case I added, --gvf is not required if --variant-peptide is not provided.