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/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java

@@ -85,6 +85,13 @@ class MockHttpServletResponseTests {
 		assertThat(response.containsHeader(headerName)).isFalse();
 	}
 
+	@Test  // gh-26493
+	void setLocaleWithNullValue() {
+		assertThat(response.getLocale()).isEqualTo(Locale.getDefault());
+		response.setLocale(null);
+		assertThat(response.getLocale()).isEqualTo(Locale.getDefault());
+	}
+
 	@Test
 	void setContentType() {
 		String contentType = "test/plain";