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

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](https://github.com/cdklabs/cdk-stacksets/blob/9c9fb653cde87378b82afc876e9784f6b9cf142b/src/stackset.ts#L260)) which will transform every empty account ID array into `undefined`.

Start in src/stackset.ts around line 260 and trace StackSetTarget.fromOrganizationalUnits(...) into the CfnStackSet deployment targets. Verify how additionalAccounts is passed through, then confirm that an empty account ID array is omitted while non-empty arrays remain unchanged and deployment validation no longer rejects the target.

Written by the indexing model from the issue text.