ES: fixing exception on branch removing

lib/elastic/indexer.rb

@@ -17,6 +17,8 @@ module Elastic
     end
 
     def run(project_id, repo_path, from_sha = nil, to_sha = nil)
+      to_sha = nil if to_sha == Gitlab::Git::BLANK_SHA
+
       vars = @vars.merge({ 'FROM_SHA' => from_sha, 'TO_SHA' => to_sha })
 
       command = ['bin/elastic_repo_indexer', project_id.to_s, repo_path]