Handle empty tagName by using Fragment

Addresses #1323. I spent hours digging this bug down. Without this little change, it was impossible to nest a custom reactpy component (i.e. a function wrapped with the component decorator). It would yield an error along the lines of "Cannot create element with

Author: shawncrawley

src/js/packages/@reactpy/client/src/vdom.tsx

@@ -93,7 +93,7 @@ function createImportSourceElement(props: {
       }
     }
   } else {
-    type = props.model.tagName;
+    type = props.model.tagName === "" ? Fragment : props.model.tagName;
   }
   return props.binding.create(
     type,