Use `undefined` instead of `null`

[bedrich-schindler]

Proposal from @bedrich-schindler and @mbohal.

In dependent projects, we often question whether null is correct default value for "empty values" and we ofter suggest to use undefined instead. This is something we would like to introduce into React UI.

Main advantage is that undefined does not propagate into DOM, while null does and it results in console errors.