Fix ExtUI compile warnings and error (#12799)

Marlin/src/gcode/bedlevel/M420.cpp

   // Enable leveling if specified, or if previously active
   set_bed_leveling_enabled(to_enable);
 
-  EXIT_M420:
+  #if HAS_MESH
+    EXIT_M420:
+  #endif
 
   // Error if leveling failed to enable or reenable
   if (to_enable && !planner.leveling_active)

Marlin/src/lcd/extensible_ui/ui_api.cpp

       pos;
 
       card.getfilename_sorted(nr);
-      return card.filename && card.filename[0] != '\0';
+      return card.filename[0] != '\0';
     #else
       return false;
     #endif
   }
 
   const char* FileList::filename() {
-    return IFSD(card.longFilename && card.longFilename[0] ? card.longFilename : card.filename, "");
+    return IFSD(card.longFilename[0] ? card.longFilename : card.filename, "");
   }
 
   const char* FileList::shortFilename() {

Marlin/src/module/temperature.h

       #endif
     #endif
 
-    #if ENABLED(ULTRA_LCD)
+    #if ENABLED(ULTRA_LCD) || ENABLED(EXTENSIBLE_UI)
       static void set_heating_message(const uint8_t e);
     #endif