[vwc-button-toggle-group] race condition between values property and child buttons in the main slot

[k-paxian]

Describe the bug

There is a bit cumbersome logic buried here and here
and this line is not allowing setting an empty array as a value w/o immediate npe 😄
and this logic needs to be re written in order to support races between properties and children's

To Reproduce

1. Set values property to some meaningful state like ['foo']
2. render the element with two buttons

    <vwc-button-toggle-group required .values=['foo']>
      <vwc-button value='foo'>Foo</vwc-button>
      <vwc-button value='bar'>Bar</vwc-button>
    </vwc-button-toggle-group>

3. when setting altogether values & childrens it's not always works as expected

Expected behavior

values property should allow idempotent usage, no matter when and how many times this property is set, the state should be consistent and independent from absence and/or presence of child buttons, since element is actually allows children mutations in runtime 😃 cite from here: One can add more buttons dynamically

Additional context
originated from this issue

[yinonov]

Thanks for the deep analysis!