Fix views sorting of NULL values as 0

When using votingapi values with views sorts, NULL values are placed at the bottom of the sort, but should rather be treated as 0 values. There are two possible solutions: (a) store zero value counts in the database (see #141439) or (b) query NULL's as 0's.

The attached patch implements the second option (b). This only changes the query used to create the views sort column. It doesn't change votingapi data storage or the calculation of values. I hope that by changing only the views sort query, that this patch will have more merit than one that changes the data structures and calculations, that would be necessary for option (a).

I've only tested it with the vote/points/sum combination, but hopefully it will work for others.