machines: Fix NIC edit for NICs without source
machines: Fix NIC edit for NICs without source

How to reproduce:
1. Attach NIC of type which doesn't require source (e.g. 'user' for
session connection VM)
2. Go to NIC Edit dialog, change NIC type to something which requires
source

Expected result: NIC configuration is changed
Actual result: Oops, Cannot read property 'remove' of undefined

Closes #13078
garrett/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,deryni/cockpit,martinpitt/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,martinpitt/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit