Return `nil` when user is not found by email.

Otherwise the sync would crash, causing all later users not being synced.

changelogs/unreleased-ee/bvl-ldap-group-sync-deleted-user.yml

+---
+title: Fix crash in LDAP sync when user was removed.
+merge_request: 2289
+author:

lib/ee/gitlab/ldap/person.rb

@@ -12,6 +12,8 @@ module EE
               found_user = adapter.user(possible_attribute, email)
               return found_user if found_user
             end
+
+            nil
           end
         end
 

spec/lib/ee/gitlab/ldap/person_spec.rb

@@ -10,15 +10,23 @@ describe Gitlab::LDAP::Person do
   end
 
   describe '.find_by_email' do
-    it 'tries finding for each configured email attribute' do
-      adapter = ldap_adapter
+    let(:adapter) { ldap_adapter }
 
+    it 'tries finding for each configured email attribute' do
       expect(adapter).to receive(:user).with('mail', 'jane@gitlab.com')
       expect(adapter).to receive(:user).with('email', 'jane@gitlab.com')
       expect(adapter).to receive(:user).with('userPrincipalName', 'jane@gitlab.com')
 
       described_class.find_by_email('jane@gitlab.com', adapter)
     end
+
+    it 'returns nil when no user was found' do
+      allow(adapter).to receive(:user)
+
+      found_user = described_class.find_by_email('jane@gitlab.com', adapter)
+
+      expect(found_user).to eq(nil)
+    end
   end
 
   describe '#kerberos_principal' do

spec/lib/gitlab/ldap/access_spec.rb

@@ -37,6 +37,12 @@ describe Gitlab::LDAP::Access, lib: true do
 
         access.allowed?
       end
+
+      context 'when looking for a user by email' do
+        let(:user) { create(:omniauth_user, external_email: true) }
+
+        it { is_expected.to be_falsey }
+      end
     end
 
     context 'when the user is found' do