The syslog standard is without encryption. So when in use you typically send this along your private management or out of band network.
Encrypted syslog is with rsyslog servers that support this typically using tls and certificates for the encryption method. Your server then is not standard syslog of encrypted rsyslog supported.
Junos configuration of this is here.
The Security Logs (security->logs->transport-protocol tls) are the only logs that are encrypted via TLS. It is unfortunate that the syslog messages (system->syslog) do not have an option for TLS transport. Clearly the device is capable of securing it as it is already being done for the Security Logs and it was deemed important to secure these messages.