[FIX] web_timeline: Make colors attribute optional (#969)

web_timeline/static/src/js/timeline_view.js

@@ -148,6 +148,8 @@ odoo.define('web_timeline.TimelineView', function (require) {
                         'value': temp.expressions[1].value
                     };
                 }).value();
+            } else {
+                this.colors = [];
             }
         },