Fix fromSource behaviour when both stdin and filename

[vrom911]

Description

While working on highlight command, I noticed that after format command changes the behaviour of the highlight command when both --stdin option specified and FILENAME is provided is changed.
Instead on returning the result for the stdin input, it started to return empty result.

This PR fixes that behaviour, it will now act exactly as before introducing the changes that came with the format command fixes.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works:
  • smoke test

[janmasrovira]

Since we are relying on this behaviour for juvix --stdin typecheck FileName.juvix in the emacs mode as well, it'd be nice to add a smoke test for that. Where we check that the file name is indeed part of the error