Edit menu enables Undo and Redo when no undo history exists

Undo and Redo are shown as available in the Edit menu even when Studio has no undo or redo work to perform, such as immediately after launching Studio and opening a solution. Keyboard shortcuts such as Cmd+Z/Ctrl+Z also fire the undo command in this state. Undo and Redo should be disabled until there is an actual document or structural action to undo or redo.