[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
Cmd can be null, see https://github.com/neovim/neovim/pull/1685#discussion_r21882874