If a project has a $0 balance ($0 in the project bank account(s)), FE would not allow the check to save and/or would give a warning that the project balance is $0 or will be negative if the check is saved. Having this warning, when creating an A/P check, could save considerable time, and prevent mistakes. This is especially true if there are many projects and a lot of activity.
You can do this in the business rules settings. Go to A/P > Configuration > Business Rules > General and set the project budget checking accordingly.