build: add SpotBugs plugin and fix all reported findings #7249

Job	Run time
sample_operators_tests (sample-operators/webpage)	4m 13s
sample_operators_tests (sample-operators/operations)	9m 49s
sample_operators_tests (sample-operators/tomcat-operator)	4m 54s
sample_operators_tests (sample-operators/leader-election)	4m 29s
sample_operators_tests (sample-operators/mysql-schema)	3m 49s
	27m 14s

Provide feedback