[FIX] web: sort with undefined value

In this awesome programming language called javascript, undefined is neither bigger nor smaller than a string. Previous code was then considering undefined values equal to any string.
This fix allows to sort in a column of a o2m field and group the undefined values together. opw 607704

addons/web/static/src/js/data.js

@@ -898,6 +898,8 @@ instance.web.BufferedDataSet = instance.web.DataSetStatic.extend({
                             sign = -1;
                             field = field.slice(1);
                         }
+                        if(!a[field] && a[field] !== 0){ return sign}
+                        if(!b[field] && b[field] !== 0){ return (sign == -1) ? 1 : -1}
                         //m2o should be searched based on value[1] not based whole value(i.e. [id, value])
                         if(_.isArray(a[field]) && a[field].length == 2 && _.isString(a[field][1])){
                             return sign * compare(a[field][1], b[field][1]);