fix: DecideBackendByRequester: lookup via get

This will make it easier to check for and deal with lookup failures.

The microservice will return a None backend instead of raising an exception,
but that's behaviour that's easier to deal with.

And if nothing else provides a default backend, SATOSA will raise KeyError: None

Author: vladimir-mencl-eresearch

src/satosa/micro_services/custom_routing.py

@@ -79,7 +79,7 @@ def process(self, context, data):
         :param context: request context
         :param data: the internal request
         """
-        context.target_backend = self.requester_mapping[data.requester]
+        context.target_backend = self.requester_mapping.get(data.requester)
         return super().process(context, data)