Adjust Event Token Validation

AdjustEventTokenValidation.swift

func checkEventToken(eventToken: String) {
    guard !ADJUtil.isNull(eventToken) else {
        logger.error("Missing Event Token")
        return false
    }
    guard eventToken.count == 6 else {
        logger.error("Malformed Event Token \(eventToken)")
        return false
    }
    return true
}