StackSetTarget - Improvement - Prevent empty account ID arrays from being passed down to the CfnStackSet

[Obirah]

When using StackSetTarget.fromOrganizationalUnits(...) and passing additionalAccounts it is possible to pass an empty array which makes sense because of the types that are necessary for that parameter (i.e. string[] | undefined).

However, the underlying CfnStackSet will cause a validation error when trying to deploy it, because it does not allow the Accounts array in the deployment target section to be empty:

The validation error: [#/StackInstancesGroup/0/DeploymentTargets/Accounts: expected minimum item count: 1, found: 0]

I'd suggest to add a check for an empty account ID array (maybe here) which will transform every empty account ID array into undefined.