Are they slin pins or luer locks or slips? It os not uncommon to have a tiny bubble at the tip and usually the fluid just flows around it. The needle goes back slightly further than the plastic so air can get caught in that little spot. You can turn the needle point down and flick it to get that bubble up to the top. Then it won't get injected it'll still be in the syringe after pinning. That said a tiny little 1IU bubble air gonna hurt you with an IM injection (or even IV for that matter)
Anyway, IM isn't gonna be an issue.
I personally always keep a little air (about 3 IU) at the top ofnthe syringe at the plunger to ensure Im not leaving goodies in the syringe. Even the little nipple plungers don't get it ALL out.