rpk redpanda start strips empty tls from config

[r-vasquez]

Version & Environment

Redpanda version: v22.1.3 (rev d1a4e00)

What went wrong?

rpk redpanda start strips out empty TLS config from redpanda.yaml.

running it with a config like

rpk:
  tls:
    truststore_file: ""
    cert_file: ""
    key_file: ""
  enable_usage_stats: true

will change to

rpk:
  enable_usage_stats: true

What should have happened instead?

If all fields are empty then it should leave the property empty instead of stripping it out:

rpk:
  tls: {}

How to reproduce the issue?

1. Stop redpanda.
2. Add empty TLS to your config file, e.g:

rpk:
  tls:
    truststore_file: ""
    cert_file: ""
    key_file: ""

3. start redpanda using rpk redpanda start
   a. This will change your config file and strip out TLS

You can do this also using a container.

Additional information

This is happening all the time when building the docker image since it takes the sample config file in https://github.com/redpanda-data/redpanda/blob/dev/conf/redpanda.yaml