Allow null in resourceUpdate()

It is not symmetric with beginPass() and endPass() otherwise (as those do
allow null).

src/rhi/qrhi.cpp

@@ -3742,7 +3742,8 @@ void QRhiResourceUpdateBatchPrivate::merge(QRhiResourceUpdateBatchPrivate *other
  */
 void QRhiCommandBuffer::resourceUpdate(QRhiResourceUpdateBatch *resourceUpdates)
 {
-    m_rhi->resourceUpdate(this, resourceUpdates);
+    if (resourceUpdates)
+        m_rhi->resourceUpdate(this, resourceUpdates);
 }
 
 /*!