obs-outputs: Attempt to generate path if one is not specified

Description

Changes the MP4 muxer to attempt to use the path generation logic if a path is not explicitly specified. This allows MP4 muxer outputs to be configured with file splitting, without requiring both the path and directory+format fields to be specified.

Motivation and Context

Currently, the UI generates the initial recording path, then lets the output do it upon file splits. While this is great, it's not very friendly to usage of the output outside of the UI (saving effort on the side of developers of custom plugins).

How Has This Been Tested?

Used a script to create a custom mp4_output output in OBS, and configured it with just directory, format, and extension fields, no path field. Output started correctly as expected, generating a path for the recording.

Types of changes

Tweak (non-breaking change to improve existing functionality)

Checklist:

[ ] My code has been run through clang-format.
[x] I have read the contributing document.
[x] My code is not on the master branch.
[x] The code has been tested.
[x] All commit messages are properly formatted and commits squashed where appropriate.
[x] I have included updates to all appropriate documentation.

obsproject / obs-studio