[Bug] Modifiers without keysets don't work as expected

### Short description

It appears to me bindings with modifiers and no keysets dont work as expected. If you bind to ctrl+alt for example it will trigger with alt only.

### How to reproduce

- `/luaui disable` to ensure no interferences
- `/bind alt+ctrl attack`
- Select a unit and press ctrl
- Notice unit has attack active command

### What was expected

Unit to not have active command, and on ctrl+alt pressed to have active command

Start by reproducing the issue with `/luaui disable`, `/bind alt+ctrl attack`, and a selected unit, then inspect the input-binding path that handles modifier-only bindings. Done means pressing ctrl alone does not activate attack, while pressing ctrl+alt does.

Written by the indexing model from the issue text.