pfe-tabs: Mutation observer logic generates error when text is added to a tab panel

[kylebuch8]

When text is dynamically added to a tab or tab panel, the mutation observer is looking for a tagName. Since text doesn't have a tagName property, it causes the mutation observer to error.

View this codesandbox to see the issue:
https://codesandbox.io/s/affectionate-snow-838pe?file=/src/App.js

[kylebuch8]

To fix the issue, we just need to make sure that the addedNode has a tagName property before continuing.

Exit here https://github.com/patternfly/patternfly-elements/blob/master/elements/pfe-tabs/src/pfe-tabs.js#L244 if the addedNode doesn't have a tagName property.