[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
>  in this case we might also pass ""

According to the function documentation on `do_call` and `call_shell`, we would need to pass `p_sh`.