Make startCursor/endCursor optional

[wilsonianb]

The pageInfo object in list responses should not require startCursor/endCursor

open-payments/open-api-spec.yaml

Lines 1278 to 1279 in 4c873db

	- startCursor
	- endCursor

They are technically required in the relay spec, but there is an open issue about making them optional (for when the result is empty), and they are currently optional in Rafiki:

• https://github.com/interledger/rafiki/blob/3d30ce755b4218a777ec77f408be5d15dfa0e687/packages/backend/src/shared/pagination.ts#L26-L30
• https://github.com/interledger/rafiki/blob/3d30ce755b4218a777ec77f408be5d15dfa0e687/packages/backend/src/graphql/schema.graphql#L134-L143

We could forgo this change by instead not having the Open Payments API list responses match relay:

• Update pagination parameters and response #136

[mkurapov]

Not required anymore in the relay spec:

We should go ahead and make them optional