Subgraph returns a null transcoder

Nobody has claimed this yet.

**Describe the bug (required)**
There's a transcoder in our transcoder list that has null fields for everything.

**Expected behavior (required)**
It shouldn't be there.

**To Reproduce (required)**
1. Go [to our subgraph](https://thegraph.com/explorer/subgraph/graphprotocol/livepeer). This query:
```graphql
{
transcoder(id: "0x0000000000000000000000000000000000000000") {
id
active
status
lastRewardRound
rewardCut
feeShare
pricePerSegment
pendingRewardCut
pendingFeeShare
pendingPricePerSegment
totalStake
}
}
```
Yields this:
```json
{
"data": {
"transcoder": {
"active": null,
"feeShare": null,
"id": "0x0000000000000000000000000000000000000000",
"lastRewardRound": null,
"pendingFeeShare": null,
"pendingPricePerSegment": null,
"pendingRewardCut": null,
"pricePerSegment": null,
"rewardCut": null,
"status": null,
"totalStake": "0"
}
}
}
```

Start by running the linked GraphQL query against the Livepeer subgraph and inspect how the transcoder with ID 0x0000000000000000000000000000000000000000 is produced. Trace the relevant subgraph mapping or entity definition, then verify that querying this zero address no longer returns a transcoder with null fields.

Written by the indexing model from the issue text.