support introspection on client credentials tokens

The response can simply omit fields that require id_token as the only
required field is "active" according to RFC 7662. I think it would be
nice for introspection of client credentials tokens to be supported by
default without needing to override
OIDC_INTROSPECTION_VALIDATE_AUDIENCE_SCOPE, but this is a start.

oidc_provider/models.py

@@ -181,7 +181,7 @@ class Token(BaseCodeTokenModel):
 
     @property
     def id_token(self):
-        return json.loads(self._id_token)
+        return json.loads(self._id_token) if self._id_token else None
 
     @id_token.setter
     def id_token(self, value):