manager: only sync label when label key exists #7818

Job	Run time
golangci-lint	4m 7s
unit-tests(Check Go modules)	25s
unit-tests(Check manifests)	43s
unit-tests(Check auto-generated codes)	2m 33s
unit-tests(Verify gofmt)	24s
unit-tests(Verify govet)	1m 2s
unit-tests(Run Go build)	1m 19s
unit-tests(Run Go test)	4m 54s
build-and-push (koordlet)	3m 31s
build-and-push (koord-manager)	2m 17s
build-and-push (koord-scheduler)	3m 0s
build-and-push (koord-descheduler)	3m 30s
	27m 45s

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