fix: prevent _entities query for `LIST_TYPE` if representations variable is empty

[PacoDu]

Gateway entities resolver should not send _entities query if there is no representations.

It shouldn't even be possible based on the definition of the query : _entities(representations: [_Any!]!): [_Entity]!. This is strange but, for now, it doesn't produce any error if the gateway sends: variables: { representations: [] } (maybe related to the _Any scalar that can be equal to undefined ?).

[PacoDu]

needs reproduction