ext6migrate: fix display of sport/dport/proto combo boxes

ext6migrate: fix display of sport/dport/proto combo boxes

setting the field value of to '' would trigger
the 'change' event which would then disable
the sport/dport/proto comboboxes too early, an empty
form would have the comboboxes disabled too

instead keep the field to a null value if it is empty, but force
it to '' before sending to the pve-proxy

also makes the macro selector keyboard editable, similar to
the other comboboxes in the input panel