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)