[GH-6629] config: Handle null attributes as invalid

[mikeblum]

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[mikeblum]

As written the omitempty is allowing the YAML to parse without an error. Added a validate_config test to factor out the yaml:omitempty issue. Should we remove the omitempty annotations to catch these sorts of issues and check for null / empty after yaml.parse or is there another way?

I could see the matrix of property checks growing quite a bit as new properties get added over time - we'd need a check for each attribute or go down the reflect route which has it's own set of issues.

https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/config/v0.3.0/generated_config.go#L427