[ButtonGroup] Fix rendering with conditional elements

[ZeeshanTamboli]

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Fixes #38978

Before: https://stackblitz.com/edit/stackblitz-starters-nykpki
After: https://codesandbox.io/s/material-ui-cra-ts-forked-4xh5j7

The issue arises for conditional elements because React.Children.count(childrenParam) also counts empty nodes, causing problems with applying the wrong positioning classes and thus resulting in incorrect styling. To avoid this, we can use React.Children.toArray, which excludes empty nodes.

I've also created a utility method to get valid React elements which is directly copied from Chakra UI source code (let me know if this is fine) so that it can be reused in other MUI packages.

(Note: This bug is also present in Joy UI ButtonGroup component.)

[mui-bot]

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[DiegoAndai]

I've also created a utility method to get valid React elements which is directly copied from Chakra UI source code (let me know if this is fine) so that it can be reused in other MUI packages.

Not sure about this one, @mnajdova what do you think?

[DiegoAndai]

Let's figure out the copy from Chakra first.

[mnajdova]

The code looks good, we don't need to include the source of where we copied it, it's not likely that we will need to backport changes in the future. The util itself looks good