Add null guards for Exec.ExecutionBuilder #3942

Job	Run time
CRD Java Models Code Generation	2m 38s
End-to-End Test Against Real Cluster	3m 50s
Java 11.0.x Maven Test on macos-latest	6m 21s
GraalVM Maven Test	3m 36s
Examples smoke test	9m 13s
Java 11.0.x Maven Test on windows-latest	4m 44s
Verify Source Format	38s
Java 11.0.x Maven Test on ubuntu-latest	3m 28s
Java 17.0.x Maven Test on macos-latest	6m 20s
Java 17.0.x Maven Test on windows-latest	5m 5s
Java 17.0.x Maven Test on ubuntu-latest	3m 35s
	49m 28s

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