fix(tree): code rebuild follow the review suggest #4

src/chart/graph/circularLayoutHelper.js

@@ -77,7 +77,7 @@ export function circularLayout(seriesModel, basedOn) {
 
     graph.eachEdge(function (edge, index) {
         var curveness = zrUtil.retrieve3(
-            edge.getModel().get('lineStyle.curveness') || null,
+            edge.getModel().get('lineStyle.curveness'),
             getCurvenessForEdge(edge, seriesModel, index),
             0
         );

src/chart/graph/forceLayout.js

@@ -86,7 +86,7 @@ export default function (ecModel) {
                 }
                 var edgeModel = edge.getModel();
                 var curveness = zrUtil.retrieve3(
-                    edge.getModel().get('lineStyle.curveness') || null,
+                    edge.getModel().get('lineStyle.curveness'),
                     -getCurvenessForEdge(edge, graphSeries, idx, true),
                     0
                 );

src/chart/graph/simpleLayoutHelper.js

@@ -40,7 +40,7 @@ export function simpleLayout(seriesModel) {
 export function simpleLayoutEdge(graph, seriesModel) {
     graph.eachEdge(function (edge, index) {
         var curveness = zrUtil.retrieve3(
-            edge.getModel().get('lineStyle.curveness') || null,
+            edge.getModel().get('lineStyle.curveness'),
             -getCurvenessForEdge(edge, seriesModel, index, true),
             0
         );