Skip noop dispatch in front-end

Workaround for Metal becuase system crashes on 0 dispatches.

Bug: dawn:640
Change-Id: I5bd33b22242ddc31816a16acb019ce2f552808bb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/39521
Commit-Queue: Hao Li <hao.x.li@intel.com>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/ComputePassEncoder.cpp

@@ -65,10 +65,14 @@ namespace dawn_native {
                 DAWN_TRY(mCommandBufferState.ValidateCanDispatch());
             }
 
+            // Skip noop dispatch. It is a workaround for system crashes on 0 dispatches on some
+            // platforms.
+            if (x != 0 && y != 0 && z != 0) {
                 DispatchCmd* dispatch = allocator->Allocate<DispatchCmd>(Command::Dispatch);
                 dispatch->x = x;
                 dispatch->y = y;
                 dispatch->z = z;
+            }
 
             return {};
         });