Specifying a range facet with a start point of 0 returns no results.

Steps to reproduce

There is a check (1147) in ezfezpsolrquerybuilder.php to ensure the start point is not empty, this rules out a value of 0:

&& !empty( $facetDefinition['range']['start'] )

This should be:

&& isset( $facetDefinition['range']['start'] )

The same probably applies for end as well, although I don't see a case where you'd want end to be 0.