Fixing air to "hasitem" will resolve the problem.
1. Hold any item like DIRT on your main hand and do the "/replaceitem entity @p slot.weapon.mainhand 1 air" command;
2. It should replace your dirt slot with air or make it an empty slot.
3. But when you do the "/testfor @p [hasitem=\{item=air,location=slot.weapon.mainhand}]" command;
4. It won't detect you even if your main hand slot is empty or air.
5. Please resolve this issue as I believe there are a lot of players out there figuring out how to detect when a player does not have an item on their main hand.

Edit: Ignore the "\" next to the "[hasitem=" It's just a bug that keeps coming back whenever I remove it.