EuiOverlayMask TS fix

[thompsongl]

Summary

Fixes children interface mismatches (below), which relates to the now-known #1890 about children element type restrictions/modifications. The type changes are not as well-defined as we'd like, but there is not good workaround yet.

Type 'Element' is not assignable to type 'string | ({} & string) | (ReactElement<any, string | ((props: any) => ReactElement<any, string | ... | (new (props: any) => Component<any, any, any>)> | null) | (new (props: any) => Component<any, any, any>)> & string) | (ReactNodeArray & string) | (ReactPortal & string) | undefined'.
  Type 'Element' is not assignable to type 'ReactPortal & string'.
    Type 'Element' is not assignable to type 'ReactPortal'.

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs
- [ ] Documentation examples were added

• A changelog entry exists and is marked appropriately

- [ ] This was checked for breaking changes and labeled appropriately
- [ ] Jest tests were updated or added to match the most common scenarios
- [ ] This was checked against keyboard-only and screenreader scenarios
- [ ] This required updates to Framer X components

[chandlerprall]

Change looks good; built eui.d.ts locally to test EuiOverlayMask usage before & after fix.

[cchaos]

@thompsongl This is now causing an error from EuiFlyout which doesn't pass any children to the overlay mask.

Maybe just make children optional?