fix: Avoid adding embedding jobs without metadata id

[Dschoordsch]

Description

When no new metadata was inserted, the resulting empty array would be fully outer joined with the models, resulting in [undefined, 'ember_1'] pairs.

Testing scenarios

• start the server and see no jobs with embed:start and embeddingsMetadataId === null is added to the EmbeddingsJobQueue table

Final checklist

• I checked the code review guidelines
• I have added Metrics Representative as reviewer(s) if my PR invovles metrics/data/analytics related changes
• I have performed a self-review of my code, the same way I'd do it for any other team member
• I have tested all cases I listed in the testing scenarios and I haven't found any issues or regressions
• Whenever I took a non-obvious choice I added a comment explaining why I did it this way
• I added the label Skip Maintainer Review Indicating the PR only requires reviewer review and can be merged right after it's approved if the PR introduces only minor changes, does not contain any architectural changes or does not introduce any new patterns and I think one review is sufficient'
• PR title is human readable and could be used in changelog

[mattkrick]

nice find! what's the culprit on why the metadata is undefined? did it previously get inserted & hit the onConflict clause?

[Dschoordsch]

It happens when nothing is inserted. The full outer join of [] and ['ember1'] is null, "ember1".