Only return query builder when the result is null for callScope().

[adrianb93]

Follow up for PR #18842.

Adrian Brown:

The way the callScope() function is implemented is to return the query builder on null, 0, or false (loose comparison) for the result.

This change will have the callScope() method on the query builder only return $this if the $result is null only.

In my application, I created a query scope that performs the query, evaluates the results and returns a boolean. I was expecting false when a query builder was returned.

Taylor Otwell:

This will need to be sent to 5.5 (master branch) since it's a breaking change.