[FIX] web: graph_renderer: check graph data before rendering

Before this commit, we had two different errors when data was
unavailable:
The first one is generated by Odoo: "No data to display, try to[...]"
The second one is generated by nvd3: "No Data Available".

We always want the same message.
So now, we always make some checks before rendering
the graph by calling _render.

addons/web/static/src/js/views/graph/graph_renderer.js

@@ -56,7 +56,7 @@ return AbstractRenderer.extend({
     on_attach_callback: function () {
         this._super.apply(this, arguments);
         this.isInDOM = true;
-        this._renderGraph();
+        this._render();
     },
     /**
      * @override

addons/web/static/tests/views/graph_tests.js

@@ -190,6 +190,25 @@ QUnit.module('Views', {
     });
 
     QUnit.test('no content helper', function (assert) {
+        assert.expect(2);
+        this.data.foo.records = [];
+
+        var graph = createView({
+            View: GraphView,
+            model: "foo",
+            data: this.data,
+            arch: '<graph string="Gloups">' +
+                        '<field name="product_id"/>' +
+                '</graph>',
+        });
+        assert.strictEqual(graph.$('div.o_graph_svg_container svg.nvd3-svg').length, 0,
+                    "should not contain a div with a svg element");
+        assert.strictEqual(graph.$('div.o_view_nocontent').length, 1,
+            "should display the no content helper");
+        graph.destroy();
+    });
+
+    QUnit.test('no content helper after update', function (assert) {
         var done = assert.async();
         assert.expect(4);