Empty psk fields are always sent when any proxy parameter is updated

Problem description: When any parameter of the proxy is updated, then the proxy.tls_psk and proxy.tls_psk_identity fields are updated with empty values. This becomes confusing for the end user once he looks into the corresponding Audit log record.
The above fields should be sent only when their values are actually updated, or when the encryption type is changed from PSK to a non-PSK encryption.

Steps to reproduce:

1. Create a passive proxy with both connections to proxy and connections from proxy set to "No encryption"
2. Update the port of this proxy
3. Check the corresponding audit log record.

Result: the corresponding audit log record shows as if the psk fields were also updated along with the proxy port:

proxy.interface: Updated
proxy.interface.port: 10051 => 10055
proxy.tls_psk: ****** => ******
proxy.tls_psk_identity: ****** => ******

Expected: If psk fields were not actually updated, then their values should not be sent, and the corresponding misleading information should not be present in audit log.