worker: Fix final scale down in one-by-one deploy
worker: Fix final scale down in one-by-one deploy

The oldScale map will most likely contain types with a zero count (i.e.
ones that have just been scaled down) and there is no need to add them to
the diff.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

lmars/flynn,flynn/flynn,philiplb/flynn,philiplb/flynn,philiplb/flynn,lmars/flynn,lmars/flynn,flynn/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,flynn/flynn