fix(Redis Streams): Allow default value of 0 for activationLagCount

[rickbrouwer]

This PR was created due to a bug found and reported as an issue #6478.

The documentation Redis Streams states that a default of 0 is allowed. In version v2.15.1 seems that be possible.
In v2.16 a check is applied with a refactoring where the value 0 is not allowed:

if r.ActivationLagCount == 0 {
	err := errors.New("activationLagCount required for Redis lag")
	return err
}

The code contains the following line:

return []external_metrics.ExternalMetricValue{metric}, metricCount > s.metadata.ActivationLagCount, nil

The second return value (the boolean) determines whether the scaler is active. This becomes true if metricCount > ActivationLagCount.
Because ActivationLagCount cannot be 0, this means that there must be at least 2 messages before scaling is activated. This means that scaling can never be activated on 1 message.

Checklist

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Fixes #6478