isisd: adjust the format of display command

isisd: adjust the format of display command

Before:
```
anlan# show isis neighbor
Area xx:
  System Id           Interface   L  State        Holdtime SNPA
 0023.0000.0000      enp1s0      2  Down          Expiring2c53.4a30.0820
```

After:
```
anlan# show isis neighbor
Area xx:
 System Id           Interface   L  State         Holdtime SNPA
 0023.0000.0000      enp1s0      2  Down          Expiring 2c53.4a30.0820
```

The `-` display format caused by no hello packet in `isis_adj_print_vty()`
is same as that of above "Expiring".

Signed-off-by: anlan_cs <anlan_cs@tom.com>