Harden accordion Javascript against accordions that have no buttons

[underdarknl]

Describe the bug, issue or concern

Accordions that exists but have not buttons currently crash the JS on the line:
buttons[0].setAttribute("aria-expanded", "true")

We should check if at least one button is available before trying to set it to expanded.

Im also not sure about the check for the aria-expanded value by just using getAttribute, as that also returns True for 'false' strings when used like this: if(elem.getAttribute('aria-expanded')) on <elem aria-explanded="false"></elem> which is not what the code expects to do on line 48 of accordion.js

To Reproduce

Steps to reproduce the behavior:

1. Create two accordions on one page
2. Make sure one has no buttons
3. Notice both have not initialized as expected.
4. See error console

Expected behavior

Init all accordions as properly as possible

Governance

• I've read the contributing document https://github.com/minvws/.github/blob/master/CONTRIBUTING.md
• I've read and understand the Code of Conduct https://github.com/minvws/.github/blob/master/CODE_OF_CONDUCT.md
• I understand that any contributions or suggestions I made may make it into the actual code. I've read the License https://github.com/minvws/nl-covid19-notification-app-coordination/blob/master/LICENSE.txt and the contributor license agreement https://github.com/minvws/nl-covid19-notification-app-coordination/blob/master/CLA.md