jqGrid show an 'edit' icon for in line editing

I am using the jqGrid with inline editing option. I want to show an edit icon if the cell does not have any values.

So I write a formatter:

 function aFormatter(cellvalue, options, row) {
        if(cellvalue == null){          
              return 'you can edit this';
        }else{
            return cellvalue;
        }
 }

The you can edit this text is displayed, when I click on it an input box is displayed correctly, however the input box as the initial value you can edit this ?

How can I fix it?

I am using the jqGrid through struts 2 jquery tags plugin, which is build on jqGrid version

I think that you should define unformatter (unformat) together with the formatter. For example,

formatter: function (cellvalue) {
   if (cellvalue == null) {          
      return "<span class='ui-icon ui-icon-pencil'></span>";
   } else {
      return cellvalue;
   };
},
unformat: function (cellValue, options, elem) {
    return $(elem).text();
}

I'm not sure how you can specify unformat in the struts2 grid plugin.

One more way would be defining formatter in the following way

(function ($) {
    "use strict";
    /*jslint unparam: true */
    $.extend($.fn.fmatter, {
        yourFormatterName: function (cellValue, options) {
            if (cellvalue == null) {          
                return "<span class='ui-icon ui-icon-pencil'></span>";
            } else {
                return cellvalue;
            };
        }
    });

    $.extend($.fn.fmatter.yourFormatterName, {
        unformat: function (cellValue, options, elem) {
            return $(elem).text();
        }
    });
}(jQuery));

It will allows you to use formatter: "yourFormatterName" (or probably formatter = "yourFormatterName" in struts2) in the same way like you can use standard formatters "integer", "date" and other.