Fix exception when data field is empty

Treat empty fields (empty data variable) as '-1' digit.
While sorting put this fields to the end/beginning.

sorting/file-size.js

@@ -29,6 +29,10 @@ jQuery.fn.dataTable.ext.type.order['file-size-pre'] = function ( data ) {
         pb: 1000000000000000
     };
 
-    var multiplier = multipliers[matches[2].toLowerCase()];
-    return parseFloat( matches[1] ) * multiplier;
+    if (matches) {
+        var multiplier = multipliers[matches[2].toLowerCase()];
+        return parseFloat( matches[1] ) * multiplier;
+    } else {
+        return -1;
+    };
 };