Allow setting empty value to some stringlist options

* eventignore
    * activate
    * newtab
    * suggestengines
    * wildoptions

Signed-off-by: Kris Maglione <maglione.k@gmail.com>

common/content/completion.js

@@ -1662,7 +1662,8 @@ function Completion() //{{{
                         completer = function () [["true", ""], ["false", ""]];
                     break;
                 case "stringlist":
-                    len = newValues.pop().length;
+                    let target = newValues.pop();
+                    len = target ? target.length : 0;
                     break;
                 case "charlist":
                     len = 0;

common/content/options.js

@@ -80,7 +80,7 @@ Option.prototype = {
     parseValues: function (value)
     {
         if (this.type == "stringlist")
-            return value.split(",");
+            return (value === "") ? [] : value.split(",");
         if (this.type == "charlist")
             return Array.slice(value);
         return value;