fix(ct): support empty fragments

[dgozman]

Currently, we ues #root vs #root > * selector for component roots depending on the number of root children. This heuristic detects fragments that render multiple elements inside the root.

However, this does not work with empty fragments that do not render anything.

The fix is to make the #root >> control=component selector that would dynamically detect the root. This supports empty fragments and also allows for dynamic updates of the fragments.

Fixes #17231.