[DialogActions] Fix allow have Children with null values

[stvkoch]

Hello

DialogActions don't allow use, common partner, that select element you want to show or send null value.

You will find this case in the test file:

<DialogActions>
  {showButton ? <button className="woof">Hello</button> : null}
  {!showButton ? <button>false button</button> : null}
</DialogActions>

Use Children.toArray to remove null/undefined values and then avoid emit error when trying access props of children.

[oliviertassinari]

That's bad for perf, can we use isValidElement instead?

[oliviertassinari]

Thanks