gitaly: Use `#blank?` to check whether the hook env is populated

The check whether the hook env is active needs to check whether the
hook env is set at all, and if it is, also whether it carries any
values. This is currently done in two calls, but can be improved to
simply use `#blank?`.

lib/gitlab/gitaly_client.rb

@@ -246,9 +246,7 @@ module Gitlab
     def self.route_to_primary
       return {} unless Gitlab::SafeRequestStore.active?
 
-      return {} unless Gitlab::SafeRequestStore[:gitlab_git_env]
-
-      return {} if Gitlab::SafeRequestStore[:gitlab_git_env].empty?
+      return {} if Gitlab::SafeRequestStore[:gitlab_git_env].blank?
 
       { 'gitaly-route-repository-accessor-policy' => 'primary-only' }
     end