Add nullability annotations to tests in module/spring-boot-kotlin-serialization

See gh-47263

module/spring-boot-kotlin-serialization/build.gradle

@@ -1,5 +1,3 @@
-import org.springframework.boot.build.autoconfigure.CheckAutoConfigurationClasses
-
 /*
  * Copyright 2012-present the original author or authors.
  *
@@ -40,3 +38,7 @@ dependencies {
 	testRuntimeOnly("ch.qos.logback:logback-classic")
 	testRuntimeOnly("org.jetbrains.kotlin:kotlin-reflect")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}