Martial arts features all require a monk to be "unarmed or only wielding monk weapons" Even if you're a kensei, that only lets you add a couple of new simple or martial weapons to your monk weapon list.

Wildshape forms all come wielding natural attacks. Natural attacks are not unarmed attacks, nor are they monk weapons, nor are they simple or martial weapons that can possibly be chosen by a kensei. Therefor, a beast is never "unarmed", and thus can never use a druid/monk's martial arts benefits even if it wanted to choose to use unarmed strikes instead of its natural attacks.

Or, can a beast choose not to "wield" it's teeth and claws? This blanket prohibition on martial arts when one has natural weapons seems overly-restrictive; it would mean, for example, that a Minotaur can never be a monk, since they are "never unarmed."