You can set the Initial Admin identity to the identity of one of your LDAP users.
The value will be dependent on how you have configured NiFi's LDAP login-identity-provider.
<property name="Identity Strategy">USE_DN</property>
If Identity Strategy is USE_DN, then the Initial Admin value should be the full DN from LDAP.
If Identity Strategy is USE_USERNAME, then the Initial Admin value should be just the username.
You may also need to shutdown NiFi, and manually remove /var/lib/nifi/conf/users.xml and /var/lib/nifi/conf/authorizations.xml from each node, because NiFi only makes use of the Initial Admin when there are no other users/groups/policies defined.