sd-network: drop fallback values

sd-network: drop fallback values

This drops spurious lines in `networkctl status` for unmanaged interfaces.
Before:
```
$ networkctl status --lines 0 lo
● 1: lo
                     Link File: n/a
                  Network File: n/a
                          Type: loopback
                         State: carrier (unmanaged)
                  Online state: unknown
                    HW Address: 00:00:00:00:00:00
                           MTU: 65536
                         QDisc: noqueue
  IPv6 Address Generation Mode: eui64
          Queue Length (Tx/Rx): 1/1
                       Address: 127.0.0.1
                                ::1
             Activation Policy: up
           Required For Online: yes
```
After:
```
$ networkctl status --lines 0 lo
● 1: lo
                     Link File: n/a
                  Network File: n/a
                         State: carrier (unmanaged)
                  Online state: unknown
                          Type: loopback
              Hardware Address: 00:00:00:00:00:00
                           MTU: 65536
                         QDisc: noqueue
  IPv6 Address Generation Mode: eui64
      Number of Queues (Tx/Rx): 1/1
                       Address: 127.0.0.1
                                ::1
```

That is, the lines for Activation Policy and Required For Online are
dropped.