Add nullability annotations to tests in module/spring-boot-http-converter

See gh-47263

module/spring-boot-http-converter/build.gradle

@@ -50,3 +50,7 @@ dependencies {
 	testRuntimeOnly("org.springframework:spring-webflux")
 	testRuntimeOnly("tools.jackson.dataformat:jackson-dataformat-cbor")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}

module/spring-boot-http-converter/src/test/java/org/springframework/boot/http/converter/autoconfigure/HttpMessageConvertersTests.java

@@ -21,6 +21,7 @@ import java.util.Collection;
 import java.util.List;
 import java.util.stream.Stream;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.converter.ByteArrayHttpMessageConverter;
@@ -172,10 +173,12 @@ class HttpMessageConvertersTests {
 
 	private List<HttpMessageConverter<?>> extractFormPartConverters(List<HttpMessageConverter<?>> converters) {
 		AllEncompassingFormHttpMessageConverter formConverter = findFormConverter(converters);
+		assertThat(formConverter).isNotNull();
 		return formConverter.getPartConverters();
 	}
 
-	private AllEncompassingFormHttpMessageConverter findFormConverter(Collection<HttpMessageConverter<?>> converters) {
+	private @Nullable AllEncompassingFormHttpMessageConverter findFormConverter(
+			Collection<HttpMessageConverter<?>> converters) {
 		for (HttpMessageConverter<?> converter : converters) {
 			if (converter instanceof AllEncompassingFormHttpMessageConverter allEncompassingConverter) {
 				return allEncompassingConverter;