Fix for bug 817497 (Generic extension icons do not display anymore in Add-ons Manager). r=dolske.

--HG--
extra : rebase_source : 3a05f0f88c72545cffd27487a2e62668d2c19550

toolkit/mozapps/extensions/content/extensions.xml

@@ -1055,7 +1055,7 @@
           if (iconURL)
             this._icon.src = iconURL;
           else
-            this._icon.src = null;
+            this._icon.src = "";
 
           if (shouldShowVersionNumber(this.mAddon))
             this._version.value = this.mAddon.version;