resolvectl: Take empty string argument to clear lists in "dns", "domain" and "nta...

resolvectl: Take empty string argument to clear lists in "dns", "domain" and "nta" options

The current CLI does not support a way to clear these lists, since without any
additional arguments, the command will list the current values.

Introduce a new way to clear the lists by passing a single '' argument to these
subcommands.

Update the man page to document this.

Tested:
  $ build/resolvectl domain eth1
  Link 3 (eth1): ~.
  $ build/resolvectl domain eth1 ''
  $ build/resolvectl domain eth1
  Link 3 (eth1):
  $ build/resolvectl domain eth1 '~.' '~example.com'
  $ build/resolvectl domain eth1
  Link 3 (eth1): ~. ~example.com
  $ build/resolvectl domain eth1 ''
  $ build/resolvectl domain eth1
  Link 3 (eth1):
  $ build/resolvectl domain eth1 '~.'
  $ build/resolvectl domain eth1
  Link 3 (eth1): ~.

And similar for "dns" and "nta".