getTransactionReceipt can return null

[area]

Only on master-arbitrum for now.

The block ingestor can (and does) recover from this, so not urgent, but for tracking purposes.

getTransactionReceipt can return null if the provided hash is not mined yet. I think this behaviour occurs when:

• A block contains no events
• When checking the receipts for each transaction in an event, we get proxied to an endpoint that hasn't seen the relevant block yet, and so get null returned.
• We try to call logs