Add nullability annotations to tests in module/spring-boot-mail

See gh-47263

module/spring-boot-mail/build.gradle

@@ -48,3 +48,11 @@ dependencies {
 
 	testRuntimeOnly("ch.qos.logback:logback-classic")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}
+
+tasks.named("compileDockerTestJava") {
+	options.nullability.checking = "tests"
+}