Fix physics being run with zero delta time (#622)

# Objective

Fixes #619.

When pausing physics, the `PhysicsSchedule` can still run even though the delta time is zero. This can cause simulation problems and sometimes even results in values becoming NaN or infinite due to division by zero.

This is a regression from 0.1, which handled scheduling differently and did not run physics with a delta time of zero.

## Solution

Don't run the `PhysicsSchedule` if the delta time is zero.

src/schedule/mod.rs

@@ -248,32 +248,35 @@ fn run_physics_schedule(world: &mut World, mut is_first_run: Local<IsFirstRun>)
             .delta()
             .mul_f64(physics_clock.relative_speed_f64());
 
-        // Advance physics clock by timestep if not paused.
+        // Advance the physics clock by the timestep if not paused.
         if !is_paused {
             world.resource_mut::<Time<Physics>>().advance_by(timestep);
 
-            // Advance substep clock already so that systems running before the substepping loop have the right delta.
+            // Advance the substep clock already so that systems running
+            // before the substepping loop have the right delta.
             let SubstepCount(substeps) = *world.resource::<SubstepCount>();
             let sub_delta = timestep.div_f64(substeps as f64);
             world.resource_mut::<Time<Substeps>>().advance_by(sub_delta);
         }
 
-        // Set generic `Time` resource to `Time<Physics>`.
+        // Set the generic `Time` resource to `Time<Physics>`.
         *world.resource_mut::<Time>() = world.resource::<Time<Physics>>().as_generic();
 
-        // Advance simulation.
-        trace!("running PhysicsSchedule");
-        schedule.run(world);
+        // Advance the simulation.
+        if !world.resource::<Time>().delta().is_zero() {
+            trace!("running PhysicsSchedule");
+            schedule.run(world);
+        }
 
-        // If physics is paused, reset delta time to stop simulation
+        // If physics is paused, reset delta time to stop the simulation
         // unless users manually advance `Time<Physics>`.
         if is_paused {
             world
                 .resource_mut::<Time<Physics>>()
                 .advance_by(Duration::ZERO);
         }
 
-        // Set generic `Time` resource back to the clock that was active before physics.
+        // Set the generic `Time` resource back to the clock that was active before physics.
         *world.resource_mut::<Time>() = old_clock;
     });