update gitlab shell secret file also when it is empty

CHANGELOG

@@ -8,6 +8,7 @@ v 8.12.0 (unreleased)
   - Prune events older than 12 months. (ritave)
   - Prepend blank line to `Closes` message on merge request linked to issue (lukehowell)
   - Filter tags by name !6121
+  - Update gitlab shell secret file also when it is empty. !3774 (glensc)
   - Give project selection dropdowns responsive width, make non-wrapping.
   - Make push events have equal vertical spacing.
   - Add two-factor recovery endpoint to internal API !5510

lib/gitlab/backend/shell.rb

@@ -195,7 +195,7 @@ module Gitlab
     # Create (if necessary) and link the secret token file
     def generate_and_link_secret_token
       secret_file = Gitlab.config.gitlab_shell.secret_file
-      unless File.exist? secret_file
+      unless File.size?(secret_file)
         # Generate a new token of 16 random hexadecimal characters and store it in secret_file.
         token = SecureRandom.hex(16)
         File.write(secret_file, token)