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

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 (saving effort on
the side of developers of custom plugins).
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f