[FIX] web_graph: no label for ungrouped pie chart

When a pie chart has no grouping selected, the label was "undefined" (the
javascript variable). Replace by the already translated string "Undefined".

Fixes #13288

addons/web_graph/static/src/js/graph_widget.js

@@ -822,7 +822,7 @@ openerp.web_graph.Graph = openerp.web.Widget.extend({
                 return p || _t('Undefined');
             }).join('/');
             if (dim_x === 0) {
-                title = self.measure_label;
+                title = self.measure_label || _t('Undefined');
             }
             return {x: title, y: self.pivot.get_total(row)[0]};
         });