Allow error-free deletion of orphaned group members

This fixes the bug where when an orphaned member
was destroyed, it errored out.

Changelog: fixed

app/models/members/group_member.rb

@@ -8,7 +8,7 @@ class GroupMember < Member
 
   belongs_to :group, foreign_key: 'source_id'
   alias_attribute :namespace_id, :source_id
-  delegate :update_two_factor_requirement, to: :user
+  delegate :update_two_factor_requirement, to: :user, allow_nil: true
 
   # Make sure group member points only to group as it source
   default_value_for :source_type, SOURCE_TYPE

spec/models/members/group_member_spec.rb

@@ -37,6 +37,10 @@ RSpec.describe GroupMember do
     end
   end
 
+  describe 'delegations' do
+    it { is_expected.to delegate_method(:update_two_factor_requirement).to(:user).allow_nil }
+  end
+
   describe '.access_level_roles' do
     it 'returns Gitlab::Access.options_with_owner' do
       expect(described_class.access_level_roles).to eq(Gitlab::Access.options_with_owner)
@@ -93,6 +97,18 @@ RSpec.describe GroupMember do
     end
   end
 
+  describe '#destroy' do
+    context 'for an orphaned member' do
+      let!(:orphaned_group_member) do
+        create(:group_member).tap { |member| member.update_column(:user_id, nil) }
+      end
+
+      it 'does not raise an error' do
+        expect { orphaned_group_member.destroy! }.not_to raise_error
+      end
+    end
+  end
+
   describe '#after_accept_invite' do
     it 'calls #update_two_factor_requirement' do
       email = 'foo@email.com'

spec/models/members/project_member_spec.rb

@@ -58,6 +58,16 @@ RSpec.describe ProjectMember do
       maintainer.destroy!
       expect(Event.recent.first).to be_left_action
     end
+
+    context 'for an orphaned member' do
+      let!(:orphaned_project_member) do
+        owner.tap { |member| member.update_column(:user_id, nil) }
+      end
+
+      it 'does not raise an error' do
+        expect { orphaned_project_member.destroy! }.not_to raise_error
+      end
+    end
   end
 
   describe '.import_team' do