mac-capture: Don't crash when migrating unknown display IDs

The current code assumes that a display UUID can be created with the
stored ID, but that's not always the case, e.g. when the user doesn't
have the display connected. As such, we need to null check this, and
fall back to the invalid ID (0) when the ID cannot be migrated.

The current code also only migrates on source creation, which yields
weird behaviour where if the user opens properties and then cancels it
would still show the first display, but only for the session. This is
why the code was factored out of the creation function and now is always
used when an ID needs to be acquired from OBS Data settings, including
when the source is updated.