[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
Ok... I guess that's a possibility thus. Although in this case we might also pass `""`, if it simplifies the special casing in `os_call_shell`.