As Adam was not born of woman so he shouldn't have had a navel. Are there any artists who didn't paint it in ?

I think it is a good point, too. But as art is artifice and so never truly true, it is unlikely that such a mythical being as Adam would be portrayed in a way that wasn't "stylised". It has now, for a while, been established that people were crucified through wrists and not hands, and we also know that Christ was not a white man. But try telling that to an artist. Myth always becomes "reality" to those who have faith and those who play God by creating. And if the belly-on was included to "complete" the man, I kind of think that he would have preferred his rib back instead, especially after all that trouble with the fruit.