Add a nil check to avoid failure when the query result is empty

Author: eheydrick

CHANGELOG.md

@@ -4,6 +4,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 This CHANGELOG follows the format listed at [Keep A Changelog](http://keepachangelog.com/)
 
 ## [Unreleased]
+### Fixed
+- metrics-postgres-query.rb: Add a nil check to avoid failure when the query result is empty (@eheydrick)
 
 ## [1.4.0] - 2017-08-04
 ### Added

bin/metrics-postgres-query.rb

@@ -114,7 +114,7 @@ def run
     value = if config[:count_tuples]
               res.ntuples
             else
-              res.first.values.first
+              res.first.values.first unless res.first.nil?
             end
 
     if config[:multirow] && !config[:count_tuples]