Simplify TLS config deserialization

[faec]

What does this PR do?

A quick cleanup to tlsClientAuth.Unpack.

Fixes #12602.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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• Start Time: 2022-04-05T20:04:59.854+0000

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[belimawr]

I only have one question/consideration.

It seems the "default" behaviour of when this config is not defined having it default to tlsClientAuthRequired is lost.

I'm not an expert on how our config lib works, but it seems that it now would return an error instead of using the default value.

[faec]

It seems the "default" behaviour of when this config is not defined having it default to tlsClientAuthRequired is lost.

That's the reason for this cleanup -- that isn't really the default behavior, but this code makes it look like it is. The in == nil branch in Unpack is never called because the actual target field is a string (and if it isn't because of a bad config, it still won't read as nil). The real value is set in libbeat/common/transport/tlscommon/server_config.go, this is just the initial config parsing.