Cloned from: https://pagure.io/freeipa/issue/9682

### Issue
ipa-migrate in stage mode fails with TypeError: 'NoneType' object is not iterable

#### Steps to Reproduce
1. ['ipa-migrate', 'stage-mode', 'master.ipa.test', '-D', 'cn=Directory Manager', '-w', 'Secret.123', '-x', '-o', '/tmp/test.ldif']

#### Actual behavior
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557 Connecting to local server ...
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557 Traceback (most recent call last):
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557   File "/usr/sbin/ipa-migrate", line 10, in <module>
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557     ipa_migrate.run()
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557   File "/usr/lib/python3.12/site-packages/ipaserver/install/ipa_migrate.py", line 2153, in run
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557     self.do_migration()
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557   File "/usr/lib/python3.12/site-packages/ipaserver/install/ipa_migrate.py", line 2040, in do_migration
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557     self.migrateDB()
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557   File "/usr/lib/python3.12/site-packages/ipaserver/install/ipa_migrate.py", line 1678, in migrateDB
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557     self.processDBOnline()
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557   File "/usr/lib/python3.12/site-packages/ipaserver/install/ipa_migrate.py", line 1645, in processDBOnline
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557     self.process_db_entry(entry_dn, entry_attrs)
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557   File "/usr/lib/python3.12/site-packages/ipaserver/install/ipa_migrate.py", line 1541, in process_db_entry
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557     self.write_update_to_ldif(local_entry)
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557   File "/usr/lib/python3.12/site-packages/ipaserver/install/ipa_migrate.py", line 625, in write_update_to_ldif
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557     for val in vals:
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557                ^^^^
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:557 TypeError: 'NoneType' object is not iterable
DEBUG    ipatests.pytest_ipa.integration.host.Host.replica0.cmd94:transport.py:217 Exit code: 1
(what happens)

#### Expected behavior
Fix the traceback

#### Version/Release/Distribution
4.13.0.dev-0.fc40

#### Additional info