Fixed issue when .removeAttribute called on a web component's convertArrayAttribute property

[dwnoble]

Fixes this error in UN staging site:

Impacts lit elements with properties with decorator:

@property({ type: Array<string>, converter: convertArrayAttribute })

When React removes a property from an element, it calls <domElement>.removeAttribute. This then calls convertArrayAttribute(undefined), which was causing an exception.

TODO: add webdriver unit test for this case