Ignore null Locale in MockHttpServletResponse

Prior to this commit, calls to setLocale() MockHttpServletResponse
would result in a NullPointerException if the supplied Locale was null.

Although the Javadoc for setLocale(Locale) and addHeader(String, String)
in javax.servlet.ServletResponse does not specify how a null
Locale should be handled, both Tomcat and Jetty simply ignore a null
value.

This commit therefore updates MockHttpServletResponse to silently
ignore a null Locale passed to setLocale().

Closes gh-26493

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java

@@ -344,6 +344,9 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setLocale(Locale locale) {
+		if (locale == null) {
+			return;
+		}
 		this.locale = locale;
 		doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, locale.toLanguageTag(), true);
 	}