Allow requestContext.authorizer be None

On Scaleway at least, authorizer is often None:

    "requestContext": {
      "accountId": "",
      "resourceId": "",
      "stage": "",
      "requestId": "",
      "resourcePath": "",
      "authorizer": None,
      "httpMethod": "GET",
      "apiId": ""
    },

serverless_wsgi.py

@@ -220,8 +220,8 @@ def handle_payload_v1(app, event, context):
         "REMOTE_ADDR": event.get("requestContext", {})
         .get("identity", {})
         .get("sourceIp", ""),
-        "REMOTE_USER": event.get("requestContext", {})
-        .get("authorizer", {})
+        "REMOTE_USER": (event.get("requestContext", {})
+                        .get("authorizer") or {})
         .get("principalId", ""),
         "REQUEST_METHOD": event.get("httpMethod", {}),
         "SCRIPT_NAME": script_name,