fix handling of empty ref_name parameter string in commits api

when params[:ref_name] is set to "" by passing an empty query parameter
to the api it is evaluated as false by the || operator.
The use of active support core extensions presence method fixes the original
implemantation.

https://guides.rubyonrails.org/active_support_core_extensions.html#presence

changelogs/unreleased/fix-commits-api-empty-refname.yml

+---
+title: Fix 500 errors in commits api caused by empty ref_name parameter
+merge_request:
+author:
+type: fixed

lib/api/commits.rb

@@ -43,7 +43,7 @@ module API
         path = params[:path]
         before = params[:until]
         after = params[:since]
-        ref = params[:ref_name] || user_project.try(:default_branch) || 'master' unless params[:all]
+        ref = params[:ref_name].presence || user_project.try(:default_branch) || 'master' unless params[:all]
         offset = (params[:page] - 1) * params[:per_page]
         all = params[:all]
         with_stats = params[:with_stats]