Add nullability annotations to tests in module/spring-boot-webflux-test

See gh-47263

module/spring-boot-webflux-test/build.gradle

@@ -39,3 +39,7 @@ dependencies {
 
 	testRuntimeOnly("ch.qos.logback:logback-classic")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}

module/spring-boot-webflux-test/src/test/java/org/springframework/boot/webflux/test/autoconfigure/ExampleIdConverter.java

@@ -20,6 +20,8 @@ import java.util.Collections;
 import java.util.Set;
 import java.util.UUID;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.convert.TypeDescriptor;
 import org.springframework.core.convert.converter.GenericConverter;
 import org.springframework.stereotype.Component;
@@ -38,7 +40,7 @@ public class ExampleIdConverter implements GenericConverter {
 	}
 
 	@Override
-	public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
+	public @Nullable Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
 		if (source == null) {
 			return null;
 		}

module/spring-boot-webflux-test/src/test/java/org/springframework/boot/webflux/test/autoconfigure/WebFluxTypeExcludeFilterTests.java

@@ -176,7 +176,7 @@ class WebFluxTypeExcludeFilterTests {
 
 		@Override
 		public Mono<Void> filter(ServerWebExchange serverWebExchange, WebFilterChain webFilterChain) {
-			return null;
+			return Mono.empty();
 		}
 
 	}