build_request: don't let set_label set a None tag
build_request: don't let set_label set a None tag

vrutkovs/osbs-client,DBuildService/osbs-client,vrutkovs/osbs-client,DBuildService/osbs-client,projectatomic/osbs-client,projectatomic/osbs-client,bfontecc007/osbs-client,bfontecc007/osbs-client