fix(server): remove thumbnailAt in asset_job_status for missing thumbnails

[mPyKen]

Similar to #12225, I realized some assets had thumbnailAt set, but had no thumbnail file in asset_files. In https://github.com/immich-app/immich/pull/11908/files#diff-99da4a5a2a767c94716e167a5d49f089778c22365cf403e92808711beb6125b9, thumbnailAt was set to NOW() if thumbnailPath IS NOT NULL, whereas asset_file was only created when "thumbnailPath" IS NOT NULL AND "thumbnailPath" != '' in https://github.com/immich-app/immich/pull/11861/files#diff-09097609a560037cd9fc82b6d5de63f184101d01c8d14a53182de0d5e649d4e0. This PR fixes actually missing thumbnails to be recognized as missing, again.

UPDATE "asset_job_status" j
SET "thumbnailAt" = NULL
WHERE j."thumbnailAt" IS NOT NULL
AND NOT EXISTS (
	SELECT 1 FROM public.asset_files f
	WHERE j."assetId" = f."assetId"
	AND f."type" = 'thumbnail'
	AND f."path" IS NOT NULL
)

[mertalev]

Ugh, I hate empty strings... The same thing applies to previews, right? It'd be best to handle them too.

[mPyKen]

Interestingly, I didn't find such cases for previews in my db. I guess we managed not to create empty strings for previews so far. I can still add that migration if need be

[mertalev]

Weirdly enough, the old preview column apparently defaulted to null while the thumbnail column defaulted to an empty string. I guess there's no need to handle it then.