fix: unhardcode ">0" values

[uri-99]

Throughout the code there are some cases where we check user's balance is >0 instead of the appropriate values.

For example:

• Batcher checks user has > 0 balance before sending task to PaymentService.
• ServiceManager.sol checks batcher has > 0 funds to pay for batch response.
• ServiceManager.sol checks batcher has > 0 funds when responding a task.