[FIX] delivery_postlogistics: raise exception when no token received

Sometimes due to API error it may happen we don't receive anything back when requesting token.
In this case we want to show meaningful message.

delivery_postlogistics/postlogistics/web_service.py

@@ -450,7 +450,20 @@ def _request_access_token(cls, delivery_carrier):
             },
             timeout=60,
         )
-        return response.json()
+
+        response.raise_for_status()
+
+        try:
+            json_response = response.json()
+        except requests.exceptions.JSONDecodeError as exc:
+            if not json_response:
+                raise exceptions.UserError(
+                    _("API response without content - no access token received.")
+                ) from exc
+            raise
+
+        _logger.debug("JSON Response: %s" % json_response)
+        return json_response
 
     @classmethod
     def get_access_token(cls, picking_carrier):