chore(network-subgraphs): Sponsorship#minOperators is not nullable (#1004)

Annotated the field as non-nullable in the GraphQL schema. The argument
type for the field is `uint` in the
`SponsorshipFactory#deploySponsorship`, and therefore it can't hold
`null` value:
```solidity
function deploySponsorship(
    uint minOperatorCount,
    string calldata streamId,
    string calldata metadata,
    address[] calldata policies,
    uint[] calldata policyParams
)
```

Author: teogeb

packages/network-subgraphs/schema.graphql

@@ -469,7 +469,7 @@ type Sponsorship @entity {
   maxOperators: Int
 
   "how many stakers are required so that Sponsorship isRunning(), i.e. starts paying"
-  minOperators: Int
+  minOperators: Int!
 
 
   ## Sponsorship state: live / updated after relevant transactions