jqGrid change formatter of cell when cell has no value

The formatter showlink produce <a> element for every input data which are strings (even empty string) or number.

I am not full sure that I understand correctly what you want.

If I understand you correctly you need to make the link "clickable" even if the cell contains empty string. To do so you can replace all empty strings in the column to something like "   ".

One more option which I can suggest you is to use my dynamicLink formatter which I described in the answer. It's very simple, but more powerful as predefined formatter showlink.

The demo shows how you can use it. The column

{ name: "mylink", width: 60, sortable: false,
    formatter: "dynamicLink",
    formatoptions: {
        cellValue: function (cellValue, rowId, rowData, options) {
            return cellValue !== "" ?
                cellValue :
                "<span style='color:red'>empty link</span>";
        },
        url: function (cellValue, rowId, rowData) {
            return '/Store/AddToCart?id=' + rowId + '?' +
                $.param({
                    name: rowData.name
                });
        }
    } }

allows to define custom cell value and the URL used in the link. The source code of the formatter you can find here. The demo displays the grid

where I placed some custom text (red text "empty link") instead of empty string.