Fix mapping when right-hand side is nil

[xaviervalarino]

When a keymap has ! on the left-hand side and an anonymous function on the right-hand side, mapargs returns a table with no rhs, which breaks the test against cmd. This only seems to be an issue for this specific lhs value.

This PR is a simple fix: it checks to make sure rhs exists before testing against cmd.

[Allaman]

AS a workaround until this is merged as Neovim is pretty much not usable without which-key:
Clone the repo and fix the line locally. With Packer you can load local plugins as well, e.g. use({ "~/workspace/github.com/which-key.nvim", config = get_config("which-key") })