Tabs behavior when activeKey is null

[taion]

We currently treat null and undefined identically for deciding whether a Tabs component is controlled: https://github.com/react-bootstrap/react-bootstrap/blob/v0.25.0/src/Tabs.js#L196

Is this what we want, or should we interpret null as "controlled, but nothing selected"?

[jquense]

I tend to prefer letting null indicate no value, vs uncontrolled. react inputs treat null the same as undefined but that works because value is a ultimately a string, so you can use '' for "empty". Most components that will be uncontrolled won't have that luxury so null is the only good option for an explicit empty value. All that to say thats why I went with allowing null in uncontrollable

[taion]

Oh, we should just use uncontrollable here. But as a quick patch, seems like we should make that check be

this.props.activeKey !== undefined

then.