subordinate_configuration in the interface expects a dict

When a subordinate charm doesn't provide an explicit value for the
optional "subordinate_configuration" arg, the error "TypeError: argument
of type 'NoneType' is not iterable" happends. Because the principal
charm expects it as a dict as documented in the interface. So let's use
a empty dict as the fallback value.

Change-Id: Ib8456ab32ec8ee5bee160d0b38a61b7ee43e16ca
Closes-Bug: #1803040

provides.py

@@ -54,6 +54,8 @@ class NeutronPluginAPISubordinate(RelationBase):
         #     }
         # }
         """
+        if subordinate_configuration is None:
+            subordinate_configuration = {}
         conversation = self.conversation()
         relation_info = {
             'neutron-plugin': neutron_plugin,