Do not set dockerfile parameter to an empty string to align with behavior of alpha controller #3260

Job	Run time
integration (v1.27.11)	20m 24s
integration (v1.27.11)	20m 24s
e2e (v1.27.11)	33m 25s
e2e (v1.27.11)	37m 22s
integration (v1.29.2)	19m 58s
integration (v1.29.2)	19m 58s
test-docker-build (base)	40s
test-docker-build (base)	40s
e2e (v1.29.2)	34m 53s
e2e (v1.29.2)	34m 53s
test-docker-build (git)	1m 5s
test-docker-build (git)	1m 5s
test-docker-build (image-processing)	50s
test-docker-build (image-processing)	50s
unit	2m 49s
unit	2m 49s
test-docker-build (waiter)	35s
test-docker-build (waiter)	35s
	3h 53m 15s

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