Prevent error in converter output when using JSON parameter

[kylelaker]

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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[david-waltermire]

@nikitawootten-nist and @wendellpiez This needs to be ported to the usnistgov/metaschema-xslt repo, since things are moving there.

[wendellpiez]

@david-waltermire-nist Indeed! Please move the Issue or instruct me in how to do so?

[nikitawootten-nist]

Superseded by usnistgov/metaschema-xslt#51, which will be a part of the upcoming OSCAL 1.1.0 release.