Don't no-op terminal.app title if it's customized

We only want to override the internal fish hardcoded title.
Fixes #3578

share/functions/__fish_config_interactive.fish

@@ -228,8 +228,9 @@ function __fish_config_interactive -d "Initializations that should be performed
         end
         if test "$TERM_PROGRAM" = "Apple_Terminal"
             # Suppress duplicative title display on Terminal.app
+            if not functions -q fish_title
                 echo -n \e\]0\;\a # clear existing title
-            function fish_title
+                function fish_title -d 'no-op terminal title'; end
             end
         end
         __update_cwd_osc # Run once because we might have already inherited a PWD from an old tab