error on creating new entry

Describe the bug

When creating a new entry phpldapadmin shows an error.

Unrecognized error number: 8192: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated

Might be associated to this issue: https://github.com/leenooks/phpLDAPadmin/issues/237

Context

• Hardware: Virtualbox VM
• YunoHost version: 11.2.5
• I have access to my server: all
• Are you in a special context or did you perform some particular tweaking on your YunoHost instance?: yes
  • If yes, please explain: Testing environment, slapd tweaked to allow full access
• Using, or trying to install package version/branch: phpLDAPadmin 1.2.6.6~ynh4

Steps to reproduce

• adapt the slapd configuration to give write permission to a user able to login
• add an entry - in my case

objectClass: inetOrgPerson, mailAccount, userPermissionYnh
ou=alias
rdn=cn

Expected behavior

show the confirmation page after hitting the "create object" button

Logs

Reproduce the failure by creating the described inetOrgPerson entry with the stated slapd permissions and phpLDAPadmin 1.2.6.6~ynh4. Start at the create-object flow and inspect the handling of the preg_match() error, using linked issue 237 for context. Done means the object is created and the confirmation page appears without the deprecation error.