Fix Error 500s creating merge requests with external issue tracker

When JIRA or Redmine were enabled and the branch name did not match the
matching regular expression, the `issue_iid` would be `nil`, preventing
users from creating merge requests.

Closes #43193

app/services/merge_requests/build_service.rb

@@ -160,10 +160,12 @@ module MergeRequests
 
       merge_request.title = "Resolve \"#{issue.title}\"" if issue.is_a?(Issue)
 
-      unless merge_request.title
-        branch_title = source_branch.downcase.remove(issue_iid.downcase).titleize.humanize
+      return if merge_request.title.present?
+
+      if issue_iid.present?
         merge_request.title = "Resolve #{issue_iid}"
-        merge_request.title += " \"#{branch_title}\"" unless branch_title.empty?
+        branch_title = source_branch.downcase.remove(issue_iid.downcase).titleize.humanize
+        merge_request.title += " \"#{branch_title}\"" if branch_title.present?
       end
     end
 

spec/services/merge_requests/build_service_spec.rb

@@ -286,15 +286,24 @@ describe MergeRequests::BuildService do
         end
       end
 
-      context 'branch starts with JIRA-formatted external issue IID' do
-        let(:source_branch) { 'EXMPL-12345' }
-
+      describe 'with JIRA enabled' do
         before do
           allow(project).to receive(:external_issue_tracker).and_return(true)
           allow(project).to receive(:issues_enabled?).and_return(false)
           allow(project).to receive(:external_issue_reference_pattern).and_return(IssueTrackerService.reference_pattern)
         end
 
+        context 'branch does not start with JIRA-formatted external issue IID' do
+          let(:source_branch) { 'test-branch' }
+
+          it 'sets the title to the humanized branch title' do
+            expect(merge_request.title).to eq('Test branch')
+          end
+        end
+
+        context 'branch starts with JIRA-formatted external issue IID' do
+          let(:source_branch) { 'EXMPL-12345' }
+
           it 'sets the title to the humanized branch title' do
             expect(merge_request.title).to eq('Resolve EXMPL-12345')
           end
@@ -316,6 +325,7 @@ describe MergeRequests::BuildService do
           end
         end
       end
+    end
 
     context 'source branch does not exist' do
       before do