The language file contains a string specifically intended for when there's not any key assigned to a certain control:

"key.keyboard.unknown": "Not bound"

Though, since 1.13-pre4, that line is not shown on controls without a key assigned to them; rather, their buttons are just empty.

Chronology:

• <17w43a (before change to LWJGL 3): The buttons used to say NONE under any language, due to a LWJGL limitation.
• 17w43a (first 1.13 snapshot, LWJGL 3 introduced): The line was in fact shown, and stayed there through the majority of the 1.13 development. Though, 1.13-pre3 was the last version to show it.
• 1.13-pre4: Line isn't shown anymore; the buttons were just made blank.