fix: encryption/decryption for non-auth plugins in consumer

[ronething]

Description

Fixes # (issue)

The data plane did not properly handle encryption and decryption when configuring a non authentication plugin on a consumer. This bug resulted in error logs and lack of encryption of sensitive information about the configuration of such plugins in etcd.

error log like:

[lua] plugin.lua:1027: decrypt_conf(): failed to get schema for plugin: csrf, context: ngx.timer"

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)