Import bitbucket issues that are reported by an anonymous user

For these kind of issues, the "reporter" field is present but zero.
In such a case, "fetch" will not return the default value, but it will
return nil.
Hence, importing fails, because the "username" field of nil is referenced

Fixes issue #44381

changelogs/unreleased/fix-bitbucket_import_anonymous.yml

+---
+title: Import bitbucket issues that are reported by an anonymous user
+merge_request: 18199
+author: bartl
+type: fixed

lib/bitbucket/representation/issue.rb

@@ -12,7 +12,7 @@ module Bitbucket
       end
 
       def author
-        raw.fetch('reporter', {}).fetch('username', nil)
+        raw.dig('reporter', 'username')
       end
 
       def description

lib/gitlab/import_formatter.rb

@@ -9,6 +9,7 @@ module Gitlab
     end
 
     def author_line(author)
+      author ||= "Anonymous"
       "*Created by: #{author}*\n\n"
     end
   end

spec/lib/gitlab/bitbucket_import/importer_spec.rb

@@ -19,6 +19,18 @@ describe Gitlab::BitbucketImport::Importer do
     ]
   end
 
+  let(:reporters) do
+    [
+      nil,
+      { "username" => "reporter1" },
+      nil,
+      { "username" => "reporter2" },
+      { "username" => "reporter1" },
+      nil,
+      { "username" => "reporter3" }
+    ]
+  end
+
   let(:sample_issues_statuses) do
     issues = []
 
@@ -36,6 +48,10 @@ describe Gitlab::BitbucketImport::Importer do
       }
     end
 
+    reporters.map.with_index do |reporter, index|
+      issues[index]['reporter'] = reporter
+    end
+
     issues
   end
 
@@ -147,5 +163,19 @@ describe Gitlab::BitbucketImport::Importer do
         expect(importer.errors).to be_empty
       end
     end
+
+    describe 'issue import' do
+      it 'maps reporters to anonymous if bitbucket reporter is nil' do
+        allow(importer).to receive(:import_wiki)
+        importer.execute
+
+        expect(project.issues.size).to eq(7)
+        expect(project.issues.where("description LIKE ?", '%Anonymous%').size).to eq(3)
+        expect(project.issues.where("description LIKE ?", '%reporter1%').size).to eq(2)
+        expect(project.issues.where("description LIKE ?", '%reporter2%').size).to eq(1)
+        expect(project.issues.where("description LIKE ?", '%reporter3%').size).to eq(1)
+        expect(importer.errors).to be_empty
+      end
+    end
   end
 end