ospfd: remove unnecessary space

Remove unnecessary spaces in config written with `interface_config_auth_str()`.

Before:
```
interface enp1s0
 ip ospf authentication  1.2.3.4
```

After:
```
interface enp1s0
 ip ospf authentication 1.2.3.4
```

Also, remove trailing one from "area <> virtual-link <> authentication"
in this same way.

Signed-off-by: anlan_cs <anlan_cs@126.com>
(cherry picked from commit 7a45f27)

Author: anlancs

ospfd/ospf_vty.c

@@ -12105,7 +12105,7 @@ static int interface_config_auth_str(struct ospf_if_params *params, char *buf)
 		break;
 
 	case OSPF_AUTH_SIMPLE:
-		snprintf(buf, BUFSIZ, " ");
+		buf[0] = '\0';
 		break;
 
 	case OSPF_AUTH_CRYPTOGRAPHIC: