stages/firewall: fix fail when setting only the default zone

When the firewall stage is provided with stage options, which set only
the default firewall zone, the `firewall-offline-cmd` command is
executed unconditionally without any parameters. This is because in this
case `ports`, `enabled_services` and `disabled_services` are all an
empty lists. This results in a failure with the following error message:

`Opening of '/etc/sysconfig/system-config-firewall' failed, exiting.`

Make sure that the second invocation of `firewall-offline-cmd` happens
conditionally, only when at least one of the `ports`, `enabled_services`
or `disabled_services` is a non-empty list.

Adjust the stage test to cover this scenario.

Signed-off-by: Tomas Hozza <thozza@redhat.com>

test/data/stages/firewall/b.json

@@ -475,6 +475,12 @@
           ]
         }
       },
+      {
+        "name": "org.osbuild.firewall",
+        "options": {
+          "default_zone": "trusted"
+        }
+      },
       {
         "name": "org.osbuild.firewall",
         "options": {
@@ -488,8 +494,7 @@
           ],
           "disabled_services": [
             "telnet"
-          ],
-          "default_zone": "trusted"
+          ]
         }
       }
     ]

test/data/stages/firewall/b.mpp.json

@@ -29,6 +29,12 @@
           }
         }
       },
+      {
+        "name": "org.osbuild.firewall",
+        "options": {
+          "default_zone": "trusted"
+        }
+      },
       {
         "name": "org.osbuild.firewall",
         "options": {
@@ -42,8 +48,7 @@
           ],
           "disabled_services": [
             "telnet"
-          ],
-          "default_zone": "trusted"
+          ]
         }
       }
     ]