MockMvcResultMatchers.forwardedUrl argument declared as nullable

Issue: SPR-17623

(cherry picked from commit 7a7958f275)

spring-test/src/main/java/org/springframework/test/web/servlet/result/MockMvcResultMatchers.java

@@ -21,6 +21,7 @@ import javax.xml.xpath.XPathExpressionException;
 
 import org.hamcrest.Matcher;
 
+import org.springframework.lang.Nullable;
 import org.springframework.test.web.servlet.ResultMatcher;
 import org.springframework.util.AntPathMatcher;
 import org.springframework.web.util.UriComponentsBuilder;
@@ -84,7 +85,7 @@ public abstract class MockMvcResultMatchers {
 	 * <p>This method accepts only exact matches.
 	 * @param expectedUrl the exact URL expected
 	 */
-	public static ResultMatcher forwardedUrl(String expectedUrl) {
+	public static ResultMatcher forwardedUrl(@Nullable String expectedUrl) {
 		return result -> assertEquals("Forwarded URL", expectedUrl, result.getResponse().getForwardedUrl());
 	}