Fix extraneous Linear Advance DIR change (#20131)

Marlin/src/module/stepper.cpp

         interval = LA_isr_rate;
       }
       else if (step_events_completed < decelerate_after && LA_current_adv_steps < LA_max_adv_steps) {
-             //step_events_completed <= (uint32_t)accelerate_until) {
         LA_steps++;
         LA_current_adv_steps++;
         interval = LA_isr_rate;
     else
       interval = LA_ADV_NEVER;
 
+    if (!LA_steps) return interval; // Leave pins alone if there are no steps!
+
     DIR_WAIT_BEFORE();
 
     #if ENABLED(MIXING_EXTRUDER)