The JavaScript needs to not attempt to do anything with the
`add_on_conf` select if it is not present in the page, as that causes
errors. For instance if the `$webworkDirs{addOnConf}` directory is not
present, or is empty.

With modern JavaScript, don't use the array `forEach` method unless it
gives a convenient and brief one liner.  Generally prefer a `for of`
loop as it is more readable. Another advantage is that `for of` loops
work on `HTMLCollection`s and `forEach` does not.  The result of a
`querySelectorAll` call or the `children` of an `Element` are
`HTMLCollection`s, and so a `for of` loop works directly, while
`Array.from` is needed to use `forEach`. This is a minor inefficiency in
this case, but that does require constructing an array from the
`HTMLCollection`.

I rewrote the `writeCourseConf` POD to clarify its usage some with the
new optional parameter.

In the `writeCourseConf` method, it should be checked that the
`$addOnConf` variable is an array reference, and not that it is not
equal to the empty string. Any numeric value or non empty string is not
equal to the empty string and would cause an error if `$addOnConf` is
set to any of those.  The check should be that `ref $addOnConf eq 'ARRAY'`.
That is the only condition that guarantees an error will not occur and
that the value of the variable will work inside the conditional block.

Minor cleanup in the `templates/ContentGenerator/CourseAdmin/add_course_form.html.ep`
file. There is no need to copy the value of `$ce->{webworkDirs}{addOnConf}`
to a local variable.  Particularly since that can be used directly even
in string interpolation.

Suggested clean up and minor issue fixes.