add fallback for fullLayout_plots :

- so that Plots.getSubplots('gl2d' or 'cartesian') does not fail
  on a blank (i.e. Plotly.plot(gd, [])) plot
- I am beginning to doubt that storing both 'cartesian' and 'gl2d'
  subplot objects in fullLayout._plots is the way to go.

Author: etpinard

src/plots/plots.js

@@ -193,7 +193,7 @@ plots.getSubplotIds = function getSubplotIds(layout, type) {
     if(type === 'cartesian' && (!layout._has || !layout._has('cartesian'))) return [];
     if(type === 'gl2d' && (!layout._has || !layout._has('gl2d'))) return [];
     if(type === 'cartesian' || type === 'gl2d') {
-        return Object.keys(layout._plots);
+        return Object.keys(layout._plots || {});
     }
 
     var idRegex = _module.idRegex,