Ensure default_branch from settings is not blank

app/models/concerns/has_repository.rb

@@ -88,7 +88,7 @@ module HasRepository
 
     group_branch_default_name = group&.default_branch_name if respond_to?(:group)
 
-    group_branch_default_name || Gitlab::CurrentSettings.default_branch_name
+    (group_branch_default_name || Gitlab::CurrentSettings.default_branch_name).presence
   end
 
   def reload_default_branch

changelogs/unreleased/fj-disallow-empty-strings-in-default-branch.yml

+---
+title: Ensure default_branch from settings is not blank
+merge_request: 49018
+author:
+type: fixed

spec/models/project_spec.rb

@@ -5071,11 +5071,11 @@ RSpec.describe Project, factory_default: :keep do
     end
   end
 
-  describe "#default_branch" do
-    context "with an empty repository" do
+  describe '#default_branch' do
+    context 'with an empty repository' do
       let_it_be(:project) { create(:project_empty_repo) }
 
-      context "group.default_branch_name is available" do
+      context 'group.default_branch_name is available' do
         let(:project_group) { create(:group) }
         let(:project) { create(:project, path: 'avatar', namespace: project_group) }
 
@@ -5088,19 +5088,19 @@ RSpec.describe Project, factory_default: :keep do
             .and_return('example_branch')
         end
 
-        it "returns the group default value" do
-          expect(project.default_branch).to eq("example_branch")
+        it 'returns the group default value' do
+          expect(project.default_branch).to eq('example_branch')
         end
       end
 
-      context "Gitlab::CurrentSettings.default_branch_name is available" do
+      context 'Gitlab::CurrentSettings.default_branch_name is available' do
         before do
           expect(Gitlab::CurrentSettings)
             .to receive(:default_branch_name)
             .and_return(example_branch_name)
         end
 
-        context "is missing or nil" do
+        context 'is missing or nil' do
           let(:example_branch_name) { nil }
 
           it "returns nil" do
@@ -5108,10 +5108,18 @@ RSpec.describe Project, factory_default: :keep do
           end
         end
 
-        context "is present" do
-          let(:example_branch_name) { "example_branch_name" }
+        context 'is blank' do
+          let(:example_branch_name) { '' }
 
-          it "returns the expected branch name" do
+          it 'returns nil' do
+            expect(project.default_branch).to be_nil
+          end
+        end
+
+        context 'is present' do
+          let(:example_branch_name) { 'example_branch_name' }
+
+          it 'returns the expected branch name' do
             expect(project.default_branch).to eq(example_branch_name)
           end
         end