[BUGFIX] EM: TimeSinceLastUpdateViewHelper fatals if never updated

If the repository was never updated (or the update failed) last_update
has the value of zero in the database.
Then NULL->format in the TimeSinceLastUpdateViewHelper leads to the fatal
error.

Resolves: #47726
Releases: 6.2, 6.1, 6.0
Change-Id: Ia41c50f2037fb1c7ef935a8855a200b303347383
Reviewed-on: https://review.typo3.org/21427
Reviewed-by: Philipp Gampe
Reviewed-by: Susanne Moog
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind