Fix #11394: Lost password email not sent when user language is invalid

When the language specified in the preferences for a user does not exist
or is otherwise invalid (for example: when updating from previous
versions of MantisBT), the lost password email is not sent to the user.

This is probably because of lang_push( user_pref_get_language( user_id )
) where user_pref_get_language can return false and lang_push expects a
string (or null).

core/lang_api.php

@@ -105,7 +105,7 @@ function lang_get_default() {
 	}
 
 	# Otherwise fall back to default
-	if( false === $t_lang ) {
+	if( null === $t_lang ) {
 		$t_lang = config_get_global( 'default_language' );
 	}
 

core/user_pref_api.php

@@ -516,15 +516,15 @@ function user_pref_get_pref( $p_user_id, $p_pref_name, $p_project_id = ALL_PROJE
  * returns user language
  * @param int $p_user_id
  * @param int $p_project_id
- * @return string
+ * @return string language name or null if invalid language specified
  */
 function user_pref_get_language( $p_user_id, $p_project_id = ALL_PROJECTS ) {
 	$t_prefs = user_pref_get( $p_user_id, $p_project_id );
 
 	// ensure the language is a valid one
 	$t_lang = $t_prefs->language;
 	if( !lang_language_exists( $t_lang ) ) {
-		$t_lang = false;
+		$t_lang = null;
 	}
 	return $t_lang;
 }