fix(Presence): add null checks on colliders before ignoring collisions

The Body Physics script would crash if no foot collider was created
because it would attempt to ignore collisions on a null object.

Author: thestonefox

Assets/VRTK/Source/Scripts/Presence/VRTK_BodyPhysics.cs

@@ -940,8 +940,14 @@ protected virtual void SetupIgnoredCollisions()
 
         protected virtual void ManagePhysicsCollider(Collider collider, bool state)
         {
-            Physics.IgnoreCollision(bodyCollider, collider, state);
-            Physics.IgnoreCollision(footCollider, collider, state);
+            if (bodyCollider != null)
+            {
+                Physics.IgnoreCollision(bodyCollider, collider, state);
+            }
+            if (footCollider != null)
+            {
+                Physics.IgnoreCollision(footCollider, collider, state);
+            }
         }
 
         protected virtual void CheckStepUpCollision(Collision collision)