[FIX] web: traceback on line graphs with only one data

Graph tooltip destruction was fixed with commit https://github.com/odoo/odoo/commit/86252428846607cd5f4b18cef8de49a8cd0b151a#diff-5de06eee7a2eeae066aa1348a1528fe9L100.
Unfortunately, the commit supposed that the 'display_' methods always
returned a graph, which is wrong.

opw-773759

addons/web/static/src/js/views/graph_widget.js

@@ -98,7 +98,9 @@ return Widget.extend({
             }));
         } else {
             var chart = this['display_' + this.mode]();
-            chart.tooltip.chartContainer(this.$el[0]);
+            if (chart) {
+                chart.tooltip.chartContainer(this.$el[0]);
+            }
         }
     },
     display_bar: function () {