Test for change to add conversion of defaultValue

This commit adds a test and polishing for a change in
AbstractNamedValueMethodArgumentResolver erroneously committed
with (unrelated) commit e57b942.

If an argument becomes null after conversion and a default value is
applied, that default value should also pass through conversion.

Closes gh-31336

Author: rstoyanchev

spring-web/src/main/java/org/springframework/web/method/annotation/AbstractNamedValueMethodArgumentResolver.java

@@ -274,9 +274,10 @@ else if (paramType.isPrimitive()) {
 		return value;
 	}
 
+	@Nullable
 	private static Object convertIfNecessary(
 			MethodParameter parameter, NativeWebRequest webRequest, WebDataBinderFactory binderFactory,
-			NamedValueInfo namedValueInfo, Object arg) throws Exception {
+			NamedValueInfo namedValueInfo, @Nullable Object arg) throws Exception {
 
 		WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
 		try {

spring-web/src/test/java/org/springframework/web/method/annotation/RequestParamMethodArgumentResolverTests.java

@@ -447,6 +447,20 @@ public void missingRequestParamEmptyValueConvertedToNull() throws Exception {
 		assertThat(arg).isNull();
 	}
 
+	@Test  // gh-31336
+	public void missingRequestParamAfterConversionWithDefaultValue() throws Exception {
+		WebDataBinder binder = new WebRequestDataBinder(null);
+
+		WebDataBinderFactory binderFactory = mock();
+		given(binderFactory.createBinder(webRequest, null, "booleanParam")).willReturn(binder);
+
+		request.addParameter("booleanParam", " ");
+
+		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(Boolean.class);
+		Object arg = resolver.resolveArgument(param, null, webRequest, binderFactory);
+		assertThat(arg).isEqualTo(Boolean.FALSE);
+	}
+
 	@Test
 	public void missingRequestParamEmptyValueNotRequired() throws Exception {
 		WebDataBinder binder = new WebRequestDataBinder(null);
@@ -687,7 +701,8 @@ public void handle(
 			@RequestParam("name") Optional<Integer> paramOptional,
 			@RequestParam("name") Optional<Integer[]> paramOptionalArray,
 			@RequestParam("name") Optional<List<?>> paramOptionalList,
-			@RequestParam("mfile") Optional<MultipartFile> multipartFileOptional) {
+			@RequestParam("mfile") Optional<MultipartFile> multipartFileOptional,
+			@RequestParam(defaultValue = "false") Boolean booleanParam) {
 	}
 
 }