LDAP: Fixed php type error when no cn provided for user

Changes default fallback for name to first DN part, otherwise the whole
DN, rather than leave as null which was causing a type error.

For #5443

tests/Auth/LdapTest.php

@@ -166,6 +166,26 @@ class LdapTest extends TestCase
         $this->assertDatabaseHas('users', ['email' => $this->mockUser->email, 'email_confirmed' => false, 'external_auth_id' => $ldapDn]);
     }
 
+    public function test_login_works_when_ldap_server_does_not_provide_a_cn_value()
+    {
+        $ldapDn = 'cn=test-user,dc=test' . config('services.ldap.base_dn');
+
+        $this->commonLdapMocks(1, 1, 1, 2, 1);
+        $this->mockLdap->shouldReceive('searchAndGetEntries')->times(1)
+            ->with($this->resourceId, config('services.ldap.base_dn'), \Mockery::type('string'), \Mockery::type('array'))
+            ->andReturn(['count' => 1, 0 => [
+                'dn'   => $ldapDn,
+                'mail' => [$this->mockUser->email],
+            ]]);
+
+        $resp = $this->mockUserLogin();
+        $resp->assertRedirect('/');
+        $this->assertDatabaseHas('users', [
+            'name' => 'test-user',
+            'email' => $this->mockUser->email,
+        ]);
+    }
+
     public function test_a_custom_uid_attribute_can_be_specified_and_is_used_properly()
     {
         config()->set(['services.ldap.id_attribute' => 'my_custom_id']);