[Feature]: Review ValueAsString return logic

[jordipuigbou]

What is your suggestion?

Review ValueAsString return logic when required value is not found. Different values are returned when not found.
Example:

	URL := golium.ValueAsString(ctx, "[CONF:url]")
	if URL == "<nil>" {
		URL = golium.ValueAsString(ctx, "[CTXT:url]")
	}
	if URL == "%nil%" {
		return "", fmt.Errorf("url shall be initialized in Configuration or Context")
	}