fix(wallet-service): we must return zero-balances for tokens the user…

[andreabadesso]

Acceptance Criteria

• To be consistent with the fullnode facade, we must return a zero-balance response for tokens the user doesn't won

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged
• Make sure either the unit tests and/or the QA tests are capable of testing the new features
• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.