macos: unsplit window shouldn't allow split zooming (#7565)

This was always the case, and is a recent regression from the SplitTree
rework. This brings it back to the previous behavior.

macos/Sources/Features/Terminal/BaseTerminalController.swift

@@ -562,6 +562,9 @@ class BaseTerminalController: NSWindowController,
             // Already zoomed, unzoom it
             surfaceTree = SplitTree(root: surfaceTree.root, zoomed: nil)
         } else {
+            // We require that the split tree have splits
+            guard surfaceTree.isSplit else { return }
+
             // Not zoomed or different node zoomed, zoom this node
             surfaceTree = SplitTree(root: surfaceTree.root, zoomed: targetNode)
         }