«وَإِنْ كَانَتْ اسْمِيَّةً فَالْمَشْهُورُ جَوَازُ تَرْكِهَا»
If the circumstantial clause is nominal, the well-known position allows wāw to be omitted.
Permission to omit does not mean that wāw is prohibited or that it is equally suitable in every construction. The discussion requires a further distinction: particular nominal clauses may favor an explicit wāw, while others have a reason for leaving it out. The exact structure and the rhetorical setting govern the choice.