Fix unexecuted CWL jobs with dynamic requirements failing on missing inputs

[adamnovak]

This will fix #4930 by patching a hole where we could run fill_in_defaults() for a CWL job's inputs before checking that its conditional was true.

Changelog Entry

To be copied to the draft changelog by merger:

• CWL jobs with dynamic requirements now have input type checking properly protected by their conditionals.

Reviewer Checklist

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