Add files existence and length check so validation is run later

[marcellamaki]

Summary

Description of the change(s) you made

Adds a simple existence check to files array used in validation.
Fixes #3642

Manual verification steps performed

1. Open to edit channel details
2. Set defaults for all options, including the important license field
3. Upload a PDF or another resource which will automatically default the Learning Activity
4. Click Finish without interacting with the edit form
5. Observe that the "error" is no longer present and the file is valid

Screenshots (if applicable)

files-validation.mp4

Does this introduce any tech-debt items?

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Reviewer guidance

How can a reviewer test these changes?

1. Open to edit channel details
2. Set defaults for all options, including the important license field
3. Upload a PDF or another resource which will automatically default the Learning Activity
4. Click Finish without interacting with the edit form
5. Observe that the "error" is no longer present and the file is valid

Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
• If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[bjester]

It seems I can still reproduce the issue, but I don't know why! I watched your recording and I didn't do anything different 🤔

[marcellamaki]

Hmmm, okay, thanks @bjester -- I'll take another look! Sorry about that 😕

[bjester]

It fixes the issue! I can't think of a way to remove the file to trigger the test case so I'm good with this.