"tabIndex" attribute is set to "0" instead of being removed

[webschik]

• Check if updating to the latest Preact version resolve the issue

Describe the bug
Hello!
I found an issue with resetting (setting to undefined) of the tabIndex attribute.
If I try to set undefined as a value for tabindex Preact sets its value to "0" instead of removing the attribute.

To Reproduce

Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/preact-tabindex-reset-fgljk?file=/src/index.js
2. Open DevTools and check the tabindex attribute on the "Test button" node
3. Click on "Switch tabindex" button
4. Check the tabindex attribute on the "Test button" node again

Expected behavior
Attribute tabindex should be removed from the DOM node.

As a workaround, I'm using a ref to a DOM node and remove the attribute in a hook... But it looks just wrong :)

P.S. Even though it's a bug report, I would like to thank all maintainers for the great library you are building!