Fix an error reporting some failures in the elasticsearch indexer

changelogs/unreleased-ee/3516-fix-nomethoderror-zero-on-nil.yml

+---
+title: Fix an error reporting some failures in the elasticsearch indexer
+merge_request: 2998
+author:
+type: fixed

lib/gitlab/elastic/indexer.rb

@@ -63,7 +63,7 @@ module Gitlab
 
         output, status = Gitlab::Popen.popen(command, nil, vars)
 
-        raise Error, output unless status.zero?
+        raise Error, output unless status&.zero?
       end
 
       def update_index_status(to_sha)