Bug: unexpected behavior around defaults when unmarshalling config

[trevorwhitney]

Describe the bug
When parsing config files, zero values resulting from the yaml unmarshalling are taking precedence over defaults.

To Reproduce
Steps to reproduce the behavior:

1. Start Loki with a config file without a server stanza
2. Expect all default values for the server config to be used
3. There is a failing test here

Expected behavior
When a block is not provided in yaml, I would expect the defaults to be used. It looks like the way we refactored parsing the default configs when introducing the dynamic config section for the common config work, that we broke this behavior, and that zero values from the yaml are being preferred over defaults.

[trevorwhitney]

I'm looking into a fix now, but if anyone has expertise in this area I'm open to idea/suggestions.

[trevorwhitney]

This turns out to be a red herring. This behavior only happens when a completely empty file is used as the config file, ie:

---

I was using that as test input which was causing the oddities, but as this is unexpected use case I don't think it needs special handling, nor do I think loki will even start if given a config like this.

[trevorwhitney]

Tests proving correctness of current behavior in #4433, not a real bug :)