updateEncryptedVersion: cleanup on target if cache already got renamed

When moving a file to trash with encryption enabled, the cache gets
moved before the actual file. According to @icewind1991 this is in order
to not break object storage.

When moving a file from an unencrypted storage (e.g. a collectives
storage) to the encrypted trashbin storage, this causes errors, see

This commit fixes it by doing `updateEncryptedVersion()` on the target
cache entry *if* the source cache entry doesn't exist anymore, but the
corresponding target cache entry does exist already.

Fixes: #26544

Signed-off-by: Jonas Meurer <jonas@freesources.org>