Fix non-working mouse wheel scrolling on the top line of the screen

Scroll the tab bar only if there actually is the tab bar, otherwise
propagate the mouse wheel event to the bufpane.

internal/action/tab.go

@@ -132,12 +132,12 @@ func (t *TabList) HandleEvent(event tcell.Event) {
 				return
 			}
 		case tcell.WheelUp:
-			if my == t.Y {
+			if my == t.Y && len(t.List) > 1 {
 				t.Scroll(4)
 				return
 			}
 		case tcell.WheelDown:
-			if my == t.Y {
+			if my == t.Y && len(t.List) > 1 {
 				t.Scroll(-4)
 				return
 			}