Remove default value from miso_genes_file to prevent checking for false file

[adamrtalbot]

The default for miso_genes_file was set to None, a Python value. It should have been null or blank. This PR removes it.

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!
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• Output Documentation in docs/output.md is updated.
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[jma1991]

Thanks @adamrtalbot for the fix, it will be merged into master in the next patch.