When the Perpendicular bisector/Bragg plane goes through the center of Ewald sphere, diffraction occurs. The perpendicular bisector to a line whose end points are on Ewald sphere but not having the integer value of h,k,l, will also go through the origin of the sphere/end of the wave vector. So why in this case diffraction will not occur?
Ewald sphere is (only) a nice tool to explain graphically Braggs law. On the other hand, Braggs law explains when scattered radiation undergoes constructive interference. For specific translation lattices this is only given at certain angles. The translation lattice transformed into a reciprocal lattice describing the lattice plane normals and not the lattice points itself defines the reciprocal lattice points and then you only have to consider Braggs law: lambda =2 dhkl sin theta, or sin theta =(lambda/2)/dhkl. This drawn into a plane is graphically the Ewald sphere. This means (ideally) that only there where you have a reciprocal lattice point (describing a lattice plane) you can expect constructive interference, i.e. intensity. Of course, the intensity is no value, your lattice is never perfect, your detectors have no point resolution etc... the detected intensity has some extended volume, but ideally it would be a point.
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