I can't speak for earlier and later versions, but in Safari 6.1.6 (on Lion) one can move even the last tab of a window into another window simply by clicking on the tab's title area and then dragging and dropping onto the tab space of your target window. Dragging the last tab to a new window causes that window to close.

This is what it looks like to move the last tab, which I had opened at Top Sites.

![enter image description here][1]


  [1]: https://i.sstatic.net/k0DJQ.png

That is actual size. The window 'collapses' during the drag operation and becomes semi-transparent.