Have the example A/B TagFilteringValve set some option again after it has set <null>

Once the value = "<null>", it stays null all the time, and never an a/b component is rendered any more.

I'll check in an initial fix, but now I get either component A, or B, or none. I do not thing we should have 'none' as an example

@Frank: Can you improve the TagFilteringValve example? Perhaps also add a small comment in it because I think it is confusing

Also I am not sure how this did work before or was it already broken?