Add support for CALL function without RETURN clause

[royi-luo]

We have some CALL functions such as the one for clearing cached warnings that change some state in the connection/DB without needing to return anything.

It would be nice to not need to return any value for these types of functions (similar to having a void return type).

So in the above example something like CALL clear_warnings() instead of needing to have CALL clear_warnings() RETURN status.