plugins/mousemark: clear drawing when no modifiers are pressed

Previously when drawing has length 1, it is not cleared. So the
remaining point in drawing can connect with the new point when the user
represses the modifiers, leading to a unexpected mark.

This ammends commit 46807b1a72.

BUG: 482297

src/plugins/mousemark/mousemark.cpp

@@ -211,8 +211,8 @@ void MouseMarkEffect::slotMouseChanged(const QPointF &pos, const QPointF &,
     } else { // neither freedraw, nor arrowdraw modifiers pressed, but mouse moved
         if (drawing.length() > 1) {
             marks.append(drawing);
-            drawing.clear();
         }
+        drawing.clear();
         arrow_tail = nullPoint();
     }
 }