s2a fix: fix NPE. #9221

Job	Run time
build(21) except self-service clients	12m 1s
build (11)	13m 23s
build(8) except for gapic-generator-java	4m 12s
build (17)	12m 46s
build(8) for gapic-generator-java	9m 32s
compatibility	1m 32s
lint	22s
showcase (11)	4m 23s
showcase (17)	4m 18s
showcase-native	8m 41s
showcase-clirr	3m 3s
showcase (21)	4m 28s
dependency_analyzer_unit_test	12s
gapic-generator-java-bom	1m 44s
unmanaged_dependency_check	5m 29s
	1h 26m 6s

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