Make interactive custom tiling more robust to suboptimal tile configs

If the root tile node contains only one child node, but the child node
has its own children, kwin will fail to show an outline when the user
interactively tiles the window.

On the other hand, there is no reason to have this child count check.
For what it's worth, it also prevents tiling the window if the output
only has one tile, which is odd, but a valid usecase.

BUG: 464379
(cherry picked from commit 76a03933)