ldap: do not store empty attribute with ldap_rfc2307_fallback_to_local_users = true
    
    This caused an error when saving local user as a fallback:
    
    ```
    [sdap_save_user] (0x0400): Storing info for user testuser@ldap.vm
    [sysdb_ldb_msg_difference] (0x2000): Added attr [gecos] to entry [name=testuser@ldap.vm,cn=users,cn=ldap.vm,cn=sysdb]
    [sysdb_set_cache_entry_attr] (0x0080): ldb_modify failed: [Invalid attribute syntax](21)[Element gecos has empty attribute in ldb message (name=testuser@ldap.vm,cn=users,cn=ldap.vm,cn=sysdb)!]
    [sysdb_set_cache_entry_attr] (0x0040): Error: 22 (Invalid argument)
    [sysdb_set_entry_attr] (0x0080): Cannot set attrs for name=testuser@ldap.vm,cn=users,cn=ldap.vm,cn=sysdb, 22 [Invalid argument]
    [sysdb_store_user] (0x0040): Cache update failed: 22
    [sysdb_store_user] (0x0400): Error: 22 (Invalid argument)
    [sdap_save_user] (0x0020): Failed to save user [testuser@ldap.vm]
    ```
    
    Steps to reproduce:
    1. create local user `testuser`
    2. add it to LDAP group memberUid
    3. set `passwd: sss files`, `group: sss files` (sss must be before files)
    4. set enable_files_domain = false and ldap_rfc2307_fallback_to_local_users = true
    5. run sssd
    6. id testuser
    -> it does not contain the LDAP group without the patch
    
    Related to:
    https://pagure.io/SSSD/sssd/issue/4013
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>