fix: ensure `totype` always returns stringified null/undefined when null/undefined is passed (#30383)

This allows the minifier to mangle the constants. It also allows the linter to find unused strings properly.

While at it, remove a few unused properties.

File                        Before      After       Diff
--------------------------------------------------------
bootstrap.bundle.min.js     23.61 kB    22.61 kB    -1.00 kB (-4.23 %)
bootstrap.min.js            17.04 kB    16.08 kB    -0.96 kB (-5.63 %)

The `prev()` function doesn't take nodes other than elements into account. Also we could simplify things a lot using the `previousElementSibling` property. This property isn't fully supported in IE, it only works for elements, but since the `element` variable is an element, we can safely use it here.

I've also added an additional test.

I don't think we had this issue in v4, since we relied on jQuery back then.

Ref. https://developer.mozilla.org/en-US/docs/Web/API/NonDocumentTypeChildNode/nextElementSibling

"without instantiate it" should be "without instantiating it"

* Close modal with keyboard=true & backdrop=static

* Enable and fix all occurrences with no-mixed-operators rule
* Take care of the max-depth warning in button.js

Signed-off-by: mhatvan <markus_hatvan@aon.at>

Dist after adding new utils file