fix for #1172

[andviro]

Hi!
Newer versions of Neovim expect the on_stdout/on_stderr handler to have the same function signature as the on_exit handler. This caused empty std_combined list and the "invalid argument" error. I've added an extra guard condition on it, just in case.

[fatih]

This is now merged with #1176. Thanks @andviro anyways 👍

[andviro]

All thanks to you, for the excellent plugin. Keep up the great work!