graph init --index-events should generate List's with non-nullable types

[evaporei]

As reported here, if graph init (with --index-events) is provided a similar ABI to this one, it will generate a schema.ts like this:

type AccessProofConditionFulfilled @entity {
  id: ID!
  _agreementId: Bytes! # bytes32
  _origHash: BigInt! # uint256
  _buyer: [BigInt]! # uint256[2]
  _provider: [BigInt]! # uint256[2]
  _cipher: [BigInt]! # uint256[2]
  _proof: Bytes! # bytes
  _conditionId: Bytes! # bytes32
}

The List types should have non-nullable members instead of nullable ones, this isn't allowed anymore since the AS update. During code generation, types such as uint256[] should be converted to [BigInt!]!.

File to change: https://github.com/graphprotocol/graph-cli/blob/master/src/scaffold/schema.js

[evaporei]

Closing because it's a copy of #808, my internet had issues and the issue got created inumerous times...