Per review feedback on #9791, the previous revision still let a
DataUpload with an empty velero.io/backup-name label fall through to
genConfigmap, creating a ConfigMap that deleteMovedSnapshots can never
match back to a snapshot. The CM is useless and only adds etcd churn.
Treat the missing-label case the same way as the foreign-owner case:
warn and skip the ConfigMap creation. Use a distinct warn message so
operators can tell the two misconfiguration classes apart in logs
(missing-label vs. owner mismatch from a captured velero namespace).
Test for the missing-label case is updated to assert no ConfigMap is
created and a warn is emitted. The warn assertion is generalized to
match the per-case message substring instead of a fixed string.
Signed-off-by: Christian Schlichtherle <christian@schlichtherle.de>