Handle nvim_{buf_}get_keymap return no rhs due to 'callback' mapping

[levouh]

This is the only issue I've run into when using the new nvim_{buf_}set_keymap functions added recently. Not sure if there is a cleaner way to fix it, but essentially we can no longer rely on the mappings having a rhs (as returned from nvim_{buf_}get_keymap) if they instead define a callback as part of the opts key.

This is the only place where I can see that this API function is used.

Fixes #224

[folke]

Thanks!