The simple answer to your question, "What will happen..." is nothing will happen. The danger of the "air bubble" is not in intramuscular injections, the danger occurs if the injection is intravenous. If too much air is injected into a vein or artery it can cause an "air embolism" or "gas embolism." Most experts agree that even if you injected a whole syringe of air into your system it wouldn't do any harm. It would take more air than a syringe can hold to cause a fatal embolism.
"Gas emboli result from the compression of respiratory gases into the blood and other tissues due to rapid changes in environmental pressure, for example, while flying or scuba diving." The amount of air a person would get in their system with pre-loaded syringes is so minor that it is not considered dangerous.