igl | vulkan | Fixed a validation warning

Summary:
```
Validation Information: [ UNASSIGNED-BestPractices-SemaphoreCount ] |
MessageID = 0x6cfe18a5 |
vkQueueSubmit():
  pSubmits[0].pWaitSemaphores is set, but pSubmits[0].waitSemaphoreCount is 0.

UNASSIGNED-BestPractices-SemaphoreCount(INFO / SPEC): msgNum: 1828591781 - Validation Information: [ UNASSIGNED-BestPractices-SemaphoreCount ] |
MessageID = 0x6cfe18a5 |
vkQueueSubmit():
  pSubmits[0].pWaitSemaphores is set, but pSubmits[0].waitSemaphoreCount is 0.
```

Reviewed By: mmaurer

Differential Revision: D52349315

fbshipit-source-id: c85d418657d3fbf87c40d7862e0891b3e97a9172

src/igl/vulkan/VulkanHelpers.c

@@ -868,7 +868,7 @@ VkSubmitInfo ivkGetSubmitInfo(const VkCommandBuffer* buffer,
   const VkSubmitInfo si = {
       .sType = VK_STRUCTURE_TYPE_SUBMIT_INFO,
       .waitSemaphoreCount = numWaitSemaphores,
-      .pWaitSemaphores = waitSemaphores,
+      .pWaitSemaphores = numWaitSemaphores ? waitSemaphores : NULL,
       .pWaitDstStageMask = waitStageMasks,
       .commandBufferCount = 1,
       .pCommandBuffers = buffer,