Allow sorting of zeros

We noticed an issue when sorting a column of numbers that had zeros for some of the values.  The zeros would always be sorted to the bottom of the column.  We looked into this and found that in lib/services/SortService.js, since 0 is falsy, values of 0 were getting caught by the logic that avoids running the sort function on null values.  The changes in this pull request allow values of 0 to be evaluated by the sort function.