Deprecate setting Parser::mTitle to null

This never happens in core code; however extensions have slipped into
a state of sin.

Bug: T235392
Change-Id: Ia254949cd8b3bc162b11dcc911dcce40d91bf1b7

RELEASE-NOTES-1.34

@@ -620,6 +620,9 @@ because of Phabricator reports.
   fetchLanguageName, getFileName, getMessagesFileName, getJsonMessagesFileName.
   Use the new LanguageNameUtils class instead. (Note that fetchLanguageName(s)
   are called getLanguageName(s) in the new class.)
+* Using the Parser without initializing its $mTitle property to non-null has
+  been deprecated.  In a future release Parser::getTitle() will throw a
+  TypeError if $mTitle is uninitialized.
 
 === Other changes in 1.34 ===
 * Added option to specify "Various authors" as author in extension credits using

includes/parser/Parser.php

@@ -228,7 +228,11 @@ class Parser {
 	public $mOptions;
 
 	/**
-	 * @var Title|null Beware - this is not always set
+	 * Since 1.34, leaving `mTitle` uninitialized or setting `mTitle` to
+	 * `null` is deprecated.
+	 *
+	 * @internal
+	 * @var Title|null
 	 */
 	public $mTitle;        # Title context, used for self-link rendering and similar things
 	public $mOutputType;   # Output type, one of the OT_xxx constants
@@ -922,9 +926,14 @@ class Parser {
 	/**
 	 * Accessor for the Title object
 	 *
+	 * Since 1.34, leaving `mTitle` uninitialized as `null` is deprecated.
+	 *
 	 * @return Title|null
 	 */
 	public function getTitle() : ?Title {
+		if ( $this->mTitle === null ) {
+			wfDeprecated( 'Parser title should never be null', '1.34' );
+		}
 		return $this->mTitle;
 	}