Save one-polygon strokes as polygons, not polylines

This solves an issue where erasing a stroke to the point that only one
(truncated) polygon was left resulted in this polygon reappearing after
reloading the document.

This should not affect any strokes containing more than one polygon.

src/domain/UBGraphicsStroke.cpp

@@ -75,9 +75,10 @@ bool UBGraphicsStroke::hasPressure()
             if (!pol->isNominalLine() || pol->originalWidth() != nominalWidth)
                 return true;
         }
+        return false;
     }
 
-    return false;
+    return true;
 }