Prevent error in output when using JSON parameter

[nikitawootten-nist]

Committer Notes

This is a recreation of @kylelaker's PR (usnistgov/metaschema#394) with some additional changes, namely correcting the XSL wrapper paths.

Previous text:

Previously, if file was not specified (and json was), an error would
be emitted about the file not being found. This was erroneous as only
the JSON input should have been considered. This applies a small fix to
remediate that, given by Wendell in usnistgov/OSCAL#1849.

I am unsure whether this is the desired long-term fix but I figured
opening a PR to memorialize the patch may be useful.

Co-authored-by: Wendell Piez wendell.piez@nist.gov

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[nikitawootten-nist]

@wendellpiez and I reviewed and tested on a call. Merging now.