🩹 Fix AUTO_FAN_PIN sanity check (#24593)

Marlin/src/inc/SanityCheck.h

@@ -2170,13 +2170,13 @@ static_assert(Y_MAX_LENGTH >= Y_BED_SIZE, "Movement bounds (Y_MIN_POS, Y_MAX_POS
  */
 #if HAS_AUTO_FAN
   #if HAS_FAN0
-    #if E0_AUTO_FAN_PIN == FAN_PIN
+    #if PIN_EXISTS(E0_AUTO_FAN) && E0_AUTO_FAN_PIN == FAN_PIN
       #error "You cannot set E0_AUTO_FAN_PIN equal to FAN_PIN."
-    #elif E1_AUTO_FAN_PIN == FAN_PIN
+    #elif PIN_EXISTS(E1_AUTO_FAN) && E1_AUTO_FAN_PIN == FAN_PIN
       #error "You cannot set E1_AUTO_FAN_PIN equal to FAN_PIN."
-    #elif E2_AUTO_FAN_PIN == FAN_PIN
+    #elif PIN_EXISTS(E2_AUTO_FAN) && E2_AUTO_FAN_PIN == FAN_PIN
       #error "You cannot set E2_AUTO_FAN_PIN equal to FAN_PIN."
-    #elif E3_AUTO_FAN_PIN == FAN_PIN
+    #elif PIN_EXISTS(E3_AUTO_FAN) &&  E3_AUTO_FAN_PIN == FAN_PIN
       #error "You cannot set E3_AUTO_FAN_PIN equal to FAN_PIN."
     #endif
   #endif