It shall be unlawful for any person to be present in any establishment where alcoholic liquors are sold or dispensed at any time during which sales of alcoholic liquors are prohibited by the foregoing sections; provided, that a person may be present in such an establishment for only fifteen minutes after the time required for cessation of sales of alcoholic liquors set out in prior §4-9 to allow the person present in the establishment at closing time to leave the premises; provided further, that the word "person" shall not include the owner or operator or his/her agent or employee when actually engaged in cleanup or custodial work. This section shall not apply to restaurants which are open for business.