Update documentation in gremlin-python driver section

When sending groovy scripts through gremlin-python client, the response from the server must be one of the primitive data types and any other return type prevents the groovy script from executing.

Florian Hockmann suggested that adding an empty array at the end of the script will help executing the script despite having an arbitrary response type and it does work.

So, I thought that this should be mentioned under the Gremlin-Python > Submitting Scripts section in the documentation.