Merge branch 'fix/commits-api-empty-refname' into 'master'

fix handling of empty ref_name parameter string in commits api

Closes #64745

See merge request gitlab-org/gitlab-ce!31687

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]

spec/requests/api/commits_spec.rb

@@ -126,6 +126,12 @@ describe API::Commits do
         end
       end
 
+      context "with empty ref_name parameter" do
+        let(:route) { "/projects/#{project_id}/repository/commits?ref_name=" }
+
+        it_behaves_like 'project commits'
+      end
+
       context "path optional parameter" do
         it "returns project commits matching provided path parameter" do
           path = 'files/ruby/popen.rb'