Yew often has loose knots that result in such holes in the limbs. The position at mid limb is a pity, but you're lucky it is positioned in the middle of the width of the limb. Personally, I would not fill it.
Since this hole is essentially an area without the full width of the limb, you need to compensate for it. You could either leave the limb thicker or wider around the knot. Judging by the looks of it, the bow seems already pretty close to final width lay-out. That means you can't make the bow wider at the knot, so you need to make it thicker. Leave a few extra rings on top of the belly around the knot, and tiller it so it is slightly stiff.