[WIP] Remove NULL/non-NULL tests after calls to vim_str(n)save

 - [x] Implement `vim_str(n)save` using `xstr(n)dup`
 - [x] Remove NULL/non-NULL tests after calls to `vim_strsave`
 - [ ] Remove NULL/non-NULL tests after calls to `vim_strnsave`
 - [ ] Remove OOM tests after calls to functions that don't have a OOM error condition after the changes to `vim_str(n)save`

ping @elmart for review