fix(dokku_network_property): check for app key (#127)

It appears that the `app` key is set in the `data` dictionary even if a value is not provided by the user.
The logic in the module assumed that the presence of the `app` key indicated that a value was provided by the user and therefore always raised an error when the `global` keyword was set to `True`.
This is fixed by checking instead whether the value for `app` is empty.

library/dokku_network_property.py

@@ -69,7 +69,7 @@ def dokku_network_property_set(data):
     has_changed = False
     meta = {"present": False}
 
-    if data["global"] and "app" in data:
+    if data["global"] and data["app"]:
         is_error = True
         meta["error"] = 'When "global" is set to true, "app" must not be provided.'
         return (is_error, has_changed, meta)