null is being removed when used as value in dangerouslySetInnerHTML html

[lfre]

Printing out null as the contents of an HTML text node through dangerouslySetInnerHTML activates the async renderer #333, and is removed from the output.

It used to be undefined but switched to null after this PR #339

E.g:

1. Correctly prints as text

<p dangerouslySetInnerHTML={{ __html: undefined }} />

2. Activates async renderer indirectly and is removed from the output.

<p dangerouslySetInnerHTML={{ __html: null }} />

[marvinhagemeister]

Makes me wonder if that is a bug overall that we accepted non-string values as the HTML string. We should do the same as in Preact itself and only allow string + number children and treat anything else as empty.