Fix potential issue on getting meta title

Author: ihorvansach

Model/Author.php

@@ -95,7 +95,7 @@ public function getMetaTitle()
             $title = $this->getTitle();
         }
 
-        return trim($title);
+        return trim($title ?: '');
     }
 
     /**

Model/Category.php

@@ -384,7 +384,7 @@ public function getMetaTitle()
             $title = $this->getData('title');
         }
 
-        return trim($title);
+        return trim($title ?: '');
     }
 
     /**

Model/Tag.php

@@ -177,7 +177,7 @@ public function getMetaTitle()
             $title = $this->getData('title');
         }
 
-        return trim($title);
+        return trim($title ?: '');
     }
 
     /**