[RFC] coverity/74718: RI: os_call_shell: handle null `cmd`

- remove invalid `FUNC_ATTR_NONNULL_ARG(1)`
- receive `(char *)` internally
- modify identifier names for consistency
- edit comments for concision and consistency
@elmart See [this hidden comment](https://github.com/neovim/neovim/pull/1685#discussion_r21882874). 

I don't know if we need to support this case anymore, I assumed it would require investigation into the interactions of our new `os_call_shell` and the old "shell expansion" and "filtering" logic. But I stopped short of investigating futher.