internal error when querying unknown subgraph with free query auth

[Theodus]

Describe the bug
Sending a query through the indexer-service, when using a free query auth token, sometimes results in an HTTP 500 error. It seems to work as expected when querying a subgraph that is being indexed.

To Reproduce

curl "${indexer_url}/subgraphs/id/${deployment}" \
  -H 'content-type: application/json' -H "authorization: Bearer ${auth_token}" \
  -d '{"query": "{ _meta { block { number } } }"}' -v

where deployment is a valid "Qm" hash for a deployment that does not exist (e.g. a deployment from testnet when running the indexer-service on mainnet).