Fallback to mirror_user for PR pipelines

Pipelines are created without an user if current_user is nil
and the pipeline triggerer is listed as API.
With this commit the pipeline triggerer will fallback to the
mirror_user that set up the project.

ee/changelogs/unreleased/32801-pipeline-by-external-pr-from-github-mirror-on-12-3-has-no-user-p.yml

+---
+title: Default current user to mirror user when creating pipelines for GitHub pull requests
+merge_request: 19072
+author:
+type: fixed

ee/lib/api/project_mirror.rb

@@ -20,7 +20,7 @@ module API
       def valid_github_signature?
         request.body.rewind
 
-        token        = project.external_webhook_token
+        token        = project.external_webhook_token.to_s
         payload_body = request.body.read
         signature    = 'sha1=' + OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha1'), token, payload_body)
 
@@ -43,7 +43,7 @@ module API
       end
 
       def process_pull_request
-        external_pull_request = ProcessGithubPullRequestEventService.new(project, current_user).execute(params)
+        external_pull_request = ProcessGithubPullRequestEventService.new(project, mirror_user).execute(params)
 
         if external_pull_request
           render_validation_error!(external_pull_request)
@@ -53,10 +53,14 @@ module API
       end
 
       def start_pull_mirroring
-        result = StartPullMirroringService.new(project, current_user).execute
+        result = StartPullMirroringService.new(project, mirror_user).execute
 
         render_api_error!(result[:message], result[:http_status]) if result[:status] == :error
       end
+
+      def mirror_user
+        current_user || project.mirror_user
+      end
     end
 
     params do