get_options: don't set optional positional params to `undef`

get_options: don't set optional positional params to `undef`

Currently this happened if (and only if) at least one
positional parameter was passed.

We run into this with
  `pmgconfig cert delete <type> [<restart>]`
vs
  `pvenode cert delete [<restart>]`

where in the PVE case the `restart` option was simply
omitted, whereas for PMG due to the existence of `<type>`
the `restart` option was explicitly passedset in the $opts
hash but ended up being `undef`.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>