Are you sure it is dFdx(p) which returns 0 and not dFdx(uv) / dFdy(uv)? We already had a problem with dFdx(uv) / dFdy(uv), see Anisotropy Issues - Worse on Mac OS - #9 by Evgeni_Popov.
The fix would be to explicitely provide tangents instead of trying to compute them with the cotangent frame.
However, your fix may well be ok, to avoid crashing the shader.