When i = r = 0 , the incident light rays are perpendicular to the surface of the interface between the two media. Light rays enter from medium1 to medium2.
Mu2 sin r = Mu1 sin i => r = 0 if and when i = 0.
When i = 90 deg, the light ray is moving along the surface of the boundary between the media. It does not enter 2nd medium. There is no question of bending.
if i=90, then r cannot be 0.