Ensure that it's possible to create accounts with zero starting balance.

[George]

With the introduction of Protocol 14 and specifically CAP-33, accounts can now be created without a starting balance when they are sponsored.

The CAP notes:

Operation Validity Changes
Any downstream system relying on the fact that CreateAccountOp is invalid if startingBalance = 0 will be affected.

This subtlety is easy to overlook, so please ensure that the SDK allows this updated constraint.

If this SDK validates the starting balance to prevent starting balances of zero, please update the SDK to allow it. When a transaction is submitted the Stellar network will ensure the amount is non-zero if it needs to be in the specific situations it is required, but SDKs should no longer require it for all cases.

[Jem Mawson]

Test added. No change required.