Document that IContextSource::getTitle can return null

That can happen if wgTitle is null.

Change-Id: I3d8aab232d408e75afb8be82ca714bf5c4fe2427

includes/context/ContextSource.php

@@ -83,7 +83,7 @@ abstract class ContextSource implements IContextSource {
 	 * Get the Title object
 	 *
 	 * @since 1.18
-	 * @return Title
+	 * @return Title|null
 	 */
 	public function getTitle() {
 		return $this->getContext()->getTitle();

includes/context/DerivativeContext.php

@@ -138,7 +138,7 @@ class DerivativeContext extends ContextSource {
 	/**
 	 * Get the Title object
 	 *
-	 * @return Title
+	 * @return Title|null
 	 */
 	public function getTitle() {
 		if ( !is_null( $this->title ) ) {

includes/context/IContextSource.php

@@ -37,7 +37,7 @@ interface IContextSource {
 	/**
 	 * Get the Title object
 	 *
-	 * @return Title
+	 * @return Title|null
 	 */
 	public function getTitle();
 

includes/context/RequestContext.php

@@ -131,7 +131,7 @@ class RequestContext implements IContextSource {
 	/**
 	 * Get the Title object
 	 *
-	 * @return Title
+	 * @return Title|null
 	 */
 	public function getTitle() {
 		if ( $this->title === null ) {