JS for back-to-top button throws error if user disabled button in config

[drammock]

You can see it on any page of MNE-Python docs, e.g. https://mne.tools/dev/

Uncaught TypeError: document.getElementById(...) is null
    <anonymous> bootstrap.js:30

The error links to webpack://pydata_sphinx_theme/src/pydata_sphinx_theme/assets/scripts/bootstrap.js line 30, AKA here:

pydata-sphinx-theme/src/pydata_sphinx_theme/assets/scripts/bootstrap.js

Lines 30 to 31 in c91e2f2

	var btn = document.getElementById("pst-back-to-top");
	btn.addEventListener("click", function () {

function needs an if(btn) guard before assigning listener