is
Finds out whether all vendors have been disallowed by the user. If, on the other hand, vendors are present, then processing will be carried out and false will be returned if at least one of them is not disallowed.
Important: If no vendor is present when calling this method no processing will be performed and null will be returned.
Return
true if all are disallowed, false if at least one vendor is not disallowed and null if no vendor is present