fix #1397 action keywords should not be set to empty string

Wox/ActionKeywords.xaml.cs

@@ -40,6 +40,7 @@ namespace Wox
         {
             var oldActionKeyword = _plugin.Metadata.ActionKeywords[0];
             var newActionKeyword = tbAction.Text.Trim();
+            newActionKeyword = newActionKeyword.Length > 0 ? newActionKeyword : "*";
             if (!PluginManager.ActionKeywordRegistered(newActionKeyword))
             {
                 var id = _plugin.Metadata.ID;