Firstly, a floating Input is really only "dangerous" if the environment is noisy AND it is tied to an IRQ or similar (in most cases).
If it isn't tied to internal hardware / IRQs, then it can't do much "damage". It's just NOT good "practice" to leave Inputs floating.
In this case, they rely on the External Clock being dynamic, and therefore the Input is really never floating. Same as if you drive
it from logic ICs etc No need for pull-up / down resistors. IF you check the specs of all I/O, you'll usually find details of how they
are set up, and what you MAY need to be careful of.