feat: gapic-generator-java to perform a no-op when no services are detected #6929

Job	Run time
build(21) except self-service clients	11m 39s
build(8) except for gapic-generator-java	4m 33s
build (11)	4m 9s
build(8) for gapic-generator-java	9m 45s
build (17)	12m 36s
compatibility	1m 32s
dependency_analyzer_unit_test	28s
gapic-generator-java-bom	2m 5s
lint	21s
showcase-clirr	2m 55s
showcase (11)	3m 57s
showcase (17)	3m 51s
showcase-native	8m 22s
unmanaged_dependency_check	3m 43s
	1h 9m 56s

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