Return an empty URL when the icon name is invalid

issue7904
review70461002

src/common.js

@@ -2762,7 +2762,7 @@
         },
         get_icon_url: function(icon_name) {
             if (!icon_name) {
-                return;
+                return jQuery.when('');
             }
             return this.register_icon(icon_name).then(function() {
                 if (icon_name in this.loaded_icons) {

src/view/form.js

@@ -865,7 +865,11 @@ function eval_pyson(value){
             }
             Sao.common.ICONFACTORY.get_icon_url(name)
                 .done(function(url) {
-                    this.img.attr('src', url);
+                    if (url) {
+                        this.img.attr('src', url);
+                    } else {
+                        this.img.removeAttr('src');
+                    }
                 }.bind(this));
         }
     });

src/view/tree.js

@@ -1570,7 +1570,11 @@
                             } else {
                                 img_tag = cell;
                             }
-                            img_tag.attr('src', url || '');
+                            if (url) {
+                                img_tag.attr('src', url);
+                            } else {
+                                img_tag.removeAttr('src');
+                            }
                         }.bind(this));
                 } else {
                     value = this.attributes.string || '';