Tree::is_empty has inconsistent behavior

[jarruda]

Describe the bug
I believe that is_empty is the intended method to know if there are any tabs present. However, it has two different behaviors that make it unsuitable for this purpose: it returns false after initialization with an empty tab set, but it returns true after adding tabs and closing them all.

To Reproduce
Steps to reproduce the behavior:

1. Initialize DockState with no tabs, it returns false because there is a single, Empty node:

DockState::new(vec![]) // calls ->

let root = Node::leaf_with(tabs);
Self {
    nodes: vec![root],
    focused_node: None,
}

2. Add a tab and close it using the UI. When the last tab is closed in the Dock, is_empty will return true because there are now no nodes in the Tree.

Expected behavior
is_empty returns true when there are no viewable tabs in the UI (or some other method fulfills this need.)