handle space-only strings, not just zero-length

IPython/html/static/notebook/js/widgets/widget_bool.js

@@ -56,7 +56,7 define(["notebook/js/widgets/widget"], function(WidgetManager){
                 this.$checkbox.prop('disabled', disabled);
 
                 var description = this.model.get('description');
-                if (description.length === 0) {
+                if (description.trim().length === 0) {
                     this.$label.hide();
                 } else {
                     this.$label.text(description);
@@ -102,7 +102,7 define(["notebook/js/widgets/widget"], function(WidgetManager){
                 this.$el.prop('disabled', disabled);
 
                 var description = this.model.get('description');
-                if (description.length === 0) {
+                if (description.trim().length === 0) {
                     this.$el.html(" "); // Preserve button height
                 } else {
                     this.$el.text(description);

IPython/html/static/notebook/js/widgets/widget_container.js

@@ -236,14 +236,14 define(["notebook/js/widgets/widget"], function(WidgetManager) {
             // Called when the model is changed.  The model may have been 
             // changed by another view or by a state update from the back-end.
             var description = this.model.get('description');
-            if (description.length === 0) {
+            if (description.trim().length === 0) {
                 this.$title.html(" "); // Preserve title height
             } else {
                 this.$title.text(description);
             }
             
             var button_text = this.model.get('button_text');
-            if (button_text.length === 0) {
+            if (button_text.trim().length === 0) {
                 this.$show_button.html(" "); // Preserve button height
             } else {
                 this.$show_button.text(button_text);