[333] Avoid to create useless StringValueStyle

Before this commit a StringValueStyle was created due to a bug in ELK
that creates an empty list of junction points.
This also causes, as side effect, an edge that seems to be "customized".

Bug: #333

plugins/org.eclipse.sirius.diagram.elk/src/org/eclipse/sirius/diagram/elk/GmfLayoutEditPolicy.java

@@ -395,7 +395,7 @@ private void addEdgeLayout(final GmfLayoutCommand command, final ElkEdge elkEdge
             // coordinates
             KVectorChain junctionPoints = elkEdge.getProperty(CoreOptions.JUNCTION_POINTS);
             String serializedJP = null;
-            if (junctionPoints != null) {
+            if (junctionPoints != null && !junctionPoints.isEmpty()) {
                 for (KVector point : junctionPoints) {
                     ElkUtil.toAbsolute(point, elkEdge.getContainingNode());
                 }