zap: always return a new child logger from *Logger.With

[liuzengh]

even if fields parameters is empty to make *Logger.With's behavior consistent with *Logger.WithOptions(Fields())'s behavior.

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[liuzengh]

previous merged refactor PR #201 “altered the external behavior of With method.

// https://github.com/uber-go/zap/pull/201/files#diff-ff87b7c4777a35588053a509583d66c9f404ccbea9e1c71d2a5f224d7ad1323eL75
func (log *logger) With(fields ...Field) Logger {
	clone := &logger{
		Meta: log.Meta.Clone(),
	}
	addFields(clone.Encoder, fields)
	return clone
}

And, I would be happy to see the behavior of *Logger.With and *Logger.WithOptions(Fields()) is same.

[rabbbit]

The PR you linked was from 2016 so likely the current state is not upsetting a lot of people.

Is this PR solving an actual issue you encountered? If so, could you elaborate on why you need the logger to be a new object?
Partially related, we were discussing this in #1340.

[liuzengh]

I guess almost no one would use With with an empty 'fields', and they don't care whether the two Loggers before and after are the same. So people haven't been upset about current state.

1. If people With an non-empty fields', it is not necessary to check whether the 'fields' is empty in the With method. This PR remove the empty 'fields' check logic, it looks good.

2. If people With with an empty 'fields'. Let's discuss two scenarios. In scenario 1, he actually doesn't want to deal with empty fields. His code implementation is flawed. Instead of using zap package to check for empty fields within the With method, he should handle the empty field checks within his business logic code. In scenario 2, he intentionally uses empty fields in the With method to get a new logger, and performs some operations on it, such as setting the log level for the new logger( implementing the SetLevel method on the wrapped zap.Logger).

Maybe, I should reword the CL "zap: always return a new child logger from *Logger.With" to "zap: Remove the empty 'fields' check logic in *Logger.With" after I see #1340.