Fix NPE in PRED__load_commit_labels_1

If a change query uses reviewer information and loads the approvals
map, but there are no approvals for a given patch set available,
the collection came out null, which cannot be iterated. Make it
always be an empty list.

Change-Id: I5fd156a854099c5d4ce3c5c0c494b5498c4ed252
1 file changed