Add NUL_STR global

Marlin/src/Marlin.cpp

 const char G28_STR[] PROGMEM = "G28",
            M21_STR[] PROGMEM = "M21",
            M23_STR[] PROGMEM = "M23 %s",
-           M24_STR[] PROGMEM = "M24";
+           M24_STR[] PROGMEM = "M24",
+           NUL_STR[] PROGMEM = "";
 
 bool Running = true;
 

Marlin/src/Marlin.h

   void event_probe_failure();
 #endif
 
-extern const char G28_STR[], M21_STR[], M23_STR[], M24_STR[];
+extern const char G28_STR[], M21_STR[], M23_STR[], M24_STR[], NUL_STR[];

Marlin/src/lcd/HD44780/ultralcd_HD44780.cpp

       // Show the Marlin logo and short build version
       // After a delay show the website URL
       //
-      logo_lines(PSTR(""));
+      extern const char NUL_STR[];
+      logo_lines(NUL_STR);
       CENTER_OR_SCROLL(SHORT_BUILD_VERSION, 1500);
       CENTER_OR_SCROLL(MARLIN_WEBSITE_URL, 1500);
       #ifdef STRING_SPLASH_LINE3

Marlin/src/lcd/extui_dgus_lcd.cpp

   void onIdle() { ScreenHandler.loop(); }
 
   void onPrinterKilled(PGM_P error, PGM_P component) {
-    ScreenHandler.sendinfoscreen(GET_TEXT(MSG_HALTED), error, PSTR(""), GET_TEXT(MSG_PLEASE_RESET), true, true, true, true);
+    extern const char NUL_STR[];
+    ScreenHandler.sendinfoscreen(GET_TEXT(MSG_HALTED), error, NUL_STR, GET_TEXT(MSG_PLEASE_RESET), true, true, true, true);
     ScreenHandler.GotoScreen(DGUSLCD_SCREEN_KILL);
     while (!ScreenHandler.loop());  // Wait while anything is left to be sent
   }

Marlin/src/lcd/menu/menu_motion.cpp

     SUBMENU(MSG_MOVE_1MM,  []{ _goto_manual_move( 1);    });
     SUBMENU(MSG_MOVE_01MM, []{ _goto_manual_move( 0.1f); });
     if (axis == Z_AXIS && (SHORT_MANUAL_Z_MOVE) > 0.0f && (SHORT_MANUAL_Z_MOVE) < 0.1f) {
-      SUBMENU_P(PSTR(""), []{ _goto_manual_move(float(SHORT_MANUAL_Z_MOVE)); });
+      extern const char NUL_STR[];
+      SUBMENU_P(NUL_STR, []{ _goto_manual_move(float(SHORT_MANUAL_Z_MOVE)); });
       MENU_ITEM_ADDON_START(0);
         char tmp[20], numstr[10];
         // Determine digits needed right of decimal

Marlin/src/module/temperature.cpp

         SERIAL_ECHOLNPGM(MSG_PID_AUTOTUNE_FINISHED);
 
         #if HAS_PID_FOR_BOTH
-          const char * const estring = GHV(PSTR("bed"), PSTR(""));
+          const char * const estring = GHV(PSTR("bed"), NUL_STR);
           say_default_(); serialprintPGM(estring); SERIAL_ECHOLNPAIR("Kp ", tune_pid.Kp);
           say_default_(); serialprintPGM(estring); SERIAL_ECHOLNPAIR("Ki ", tune_pid.Ki);
           say_default_(); serialprintPGM(estring); SERIAL_ECHOLNPAIR("Kd ", tune_pid.Kd);