fix: Assignment does not validate account number if target type is unset

The default is that the target type is an account so we should run validation if the prop is undefined.

src/assignment.ts

@@ -82,7 +82,7 @@ export class Assignment extends AssignmentBase {
   constructor(scope: Construct, id: string, props: AssignmentProps) {
     super (scope, id);
 
-    if (props.targetType === TargetTypes.AWS_ACCOUNT) {
+    if (props.targetType === TargetTypes.AWS_ACCOUNT || props.targetType === undefined) {
       Assignment.validateAwsAccountTargetId(props.targetId);
     }
     validatePrincipal(props.principal);