Removed un-needed tests.

jQuery `.text()` function always returns a string

src/extensions/filter-control/bootstrap-table-filter-control.js

@@ -72,8 +72,6 @@
             $opts.sort(function(a,b){
                 a = $(a).text().toLowerCase();
                 b = $(b).text().toLowerCase();
-                if (a === undefined || a === null) { a = ''; }
-                if (b === undefined || b === null) { b = ''; }
                 if ($.isNumeric(a) && $.isNumeric(b)) {
                     // Convert numerical values from string to float.
                     a = parseFloat(a);