Fix applying suggestions with empty custom message

When the custom suggestion message is cleared,
it's stored as an empty string in db. In this case
we still need to apply default commit message

app/services/suggestions/apply_service.rb

@@ -96,7 +96,7 @@ module Suggestions
     end
 
     def suggestion_commit_message(project)
-      project.suggestion_commit_message || DEFAULT_SUGGESTION_COMMIT_MESSAGE
+      project.suggestion_commit_message.presence || DEFAULT_SUGGESTION_COMMIT_MESSAGE
     end
 
     def processed_suggestion_commit_message(suggestion)

changelogs/unreleased/id-fix-suggestions.yml

+---
+title: Fix applying the suggestions with an empty custom message
+merge_request: 24144
+author:
+type: fixed

spec/services/suggestions/apply_service_spec.rb

@@ -57,10 +57,22 @@ describe Suggestions::ApplyService do
       end
 
       context 'is not specified' do
-        let(:message) { nil }
+        let(:expected_value) { "Apply suggestion to files/ruby/popen.rb" }
 
-        it 'sets default commit message' do
-          expect(project.repository.commit.message).to eq("Apply suggestion to files/ruby/popen.rb")
+        context 'is nil' do
+          let(:message) { nil }
+
+          it 'sets default commit message' do
+            expect(project.repository.commit.message).to eq(expected_value)
+          end
+        end
+
+        context 'is an empty string' do
+          let(:message) { '' }
+
+          it 'sets default commit message' do
+            expect(project.repository.commit.message).to eq(expected_value)
+          end
         end
       end