Fix compiler warning (#15642)

Marlin/planner.cpp

@@ -1169,8 +1169,11 @@ void Planner::recalculate() {
  * Maintain fans, paste extruder pressure,
  */
 void Planner::check_axes_activity() {
-  unsigned char axis_active[NUM_AXIS] = { 0 },
-                tail_fan_speed[FAN_COUNT];
+  uint8_t axis_active[NUM_AXIS] = { 0 };
+
+  #if FAN_COUNT > 0
+    uint8_t tail_fan_speed[FAN_COUNT] = { 0 };
+  #endif
 
   #if ENABLED(BARICUDA)
     #if HAS_HEATER_1