fix animateContentSize NavHost Modifier

[tomczyn]

Changed NavHost layout implementation so that BoxWithConstraints layout that wraps the NavHostContent will be composed only when there's a BackStackEntry to display.

There is an issue when the NavHost is inside a ModalBottomSheet, and the NavHost is wrapped by animateContentSize() Modifier. It interferes with showing animation of the bottom sheet, where it hangs for an additional ~600ms at the bottom of the screen. It appears that not composing the layout (BoxWithConstraints) when there's no BackStackEntry solves the issue.

Issue link: #313

Changes:

• Added an if statement wrapping BoxWithConstraints so that it'll compose only when currentSceneEntry or currentFloatingEntry is not null.

We could avoid the if if not for the floating BackStackEntry which is also wrapped by the same BoxWithConstraints layout. If you have any idea how we could avoid it in some other way, feel free to update the PR with new changes. Or instead of nesting the rest of the logic inside the if we could invert it and do a return so the rest of the function won't run.

[tomczyn]

this could be inverted if you prefer to avoid nesting, e.g.

if (currentSceneEntry == null && currentFloatingEntry == null) return

[Tlaster]

Thanks for the PR! LGTM!