set up tls configuration

[Andrei Katsuk]

Hi,
I want to  set
ca_file in the tls config section to env variable as follows:
tls {
    ca_file = $ENV{LDAP_CERT}
}
and skip it if  $ENV{LDAP_CERT} is empty.
If it is empty FreeRADIUS shows next error:
===========
Unable to check file "": No such file or directory
 /etc/raddb/mods-enabled/ldap[488]: Failed parsing configuration item "ca_file"
================
How can I skip it in this case?

Andrei