There's nothing you can eat the night before that will make a difference. The blood sugar cycle is measured in minutes and hours, so what you eat now is in your bloodstream very quickly, depending on how full your stomach is and what you eat with it, then out of the bloodstream equally quickly.

If you are hypoglycemic, you're probably prone to sugar spikes immediately after eating, and feel the effects of the hypoglycemia afterward.

So, ride slower and easier to avoid drawing your limited reserves down, and get there early to allow you to recover and stabilize before the blood is drawn.

OTOH - if you normally feel the effects of hypoglycemia after rides this short, then you might eat a small chocolate bar half an hour before starting out. But be sure to give your doctor a heads up either way, so he can advise otherwise, or at least factor it when looking at the test results.