Make JsonPathAssertions#isEqualTo parameter nullable

Closes gh-35445

Author: sdeleuze

spring-test/src/main/java/org/springframework/test/web/reactive/server/JsonPathAssertions.java

@@ -56,7 +56,7 @@ public class JsonPathAssertions {
 	/**
 	 * Applies {@link JsonPathExpectationsHelper#assertValue(String, Object)}.
 	 */
-	public WebTestClient.BodyContentSpec isEqualTo(Object expectedValue) {
+	public WebTestClient.BodyContentSpec isEqualTo(@Nullable Object expectedValue) {
 		this.pathHelper.assertValue(this.content, expectedValue);
 		return this.bodySpec;
 	}

spring-test/src/test/java/org/springframework/test/util/JsonPathExpectationsHelperTests.java

@@ -57,7 +57,8 @@ class JsonPathExpectationsHelperTests {
 				'whitespace':  '    ',
 				'emptyString': '',
 				'emptyArray':  [],
-				'emptyMap':    {}
+				'emptyMap':    {},
+				'nullValue':   null
 			}""";
 
 	private static final String SIMPSONS = """
@@ -249,6 +250,11 @@ void assertValue() {
 		new JsonPathExpectationsHelper("$.num").assertValue(CONTENT, 5);
 	}
 
+	@Test
+	void assertNullValue() {
+		new JsonPathExpectationsHelper("$.nullValue").assertValue(CONTENT, (Object) null);
+	}
+
 	@Test // SPR-14498
 	void assertValueWithNumberConversion() {
 		new JsonPathExpectationsHelper("$.num").assertValue(CONTENT, 5.0);