Set default frame time on missing video duration metadata

[DurandA]

PR Goal

Prevent exception when generating a preview on a video with missing duration metadata. duration metadata is typically missing on webm files recorded using the MediaStream Recording API. There is even a JS package to add duration to webm files.

Implemented solution

If there is no duration metadata, take the first frame rather than throwing a KeyError.

Checkpoints

• If relevant, manual tests have been done to ensure the stability of the whole application and that the involved feature works
• Automated tests covering the feature or the fix, have been written, deemed irrelevant (give the reason), or an issue has been created to implement the test (give the link)
• The PR or the original issue contains a short summary of the implemented solution.