(defclass foo () ((bar :initarg := :initform '())))

(defmethod make-instance :around ((type (eql 'foo)) &key =)
    (if (not =) nil (call-next-method)))

(print (make-instance 'foo))    ;; => NIL

(print (let ((x (make-instance 'foo))) x)) ;; => #<FOO {10037EEDF3}>

(print (let ((x (apply #'make-instance (list 'foo)))) x)) ;; => NIL