MTG: Fiend Binder and wider implications of Rule 508.1d
You're getting combat requirements mixed up with triggered abilities.
Triggered abilities:

603.1. Triggered abilities have a trigger condition and an effect. They are written as “[When/Whenever/At] [trigger condition or event], [effect]. [Instructions (if any).]”

Now, you know that if there isn't any targets, there's a problem, but that's handled by:

603.3d The remainder of the process for putting a triggered ability on the stack is identical to the process for casting a spell listed in rules 601.2c–d. If a choice is required when the triggered ability goes on the stack but no legal choices can be made for it, or if a rule or a continuous effect otherwise makes the ability illegal, the ability is simply removed from the stack.

So, if your opponent has no creatures, the ability triggers, but since there are no legal targets, it's immediately removed from the stack.