Selectlist without enabled options throws error

A selectlist without options or with disabled options only will throw `Cannot set properties of undefined (setting 'tabIndex')` error. [Selectlist expects to have at least one enabled input.](https://github.com/luwes/selectlist-polyfill/blob/b962b6e84476ebb453002388de2c05cd58e2ea95/src/selectlist.js#L441)
[Reproduction](https://stackblitz.com/edit/web-platform-t65p3h?file=index.html) - click on the selectlist.

Expected behavior:
It should open just an empty popover? 🤔 Not sure about the specs, the select opens an empty list of options.

Start at src/selectlist.js around line 441 and reproduce the error using the linked StackBlitz example, especially with no options and with only disabled options. Check the selectlist specification to determine the expected empty-popover behavior, then confirm the fix no longer throws when the list has no enabled inputs.

Written by the indexing model from the issue text.