CloudwatchLogsRawData.Parse() Handle Non-Base64

When sending data from a Cloudwatch Log Destination => AWS Kinesis, and the Lambda function uses the following signature:

```
func HandleCloudWatchLogs(ctx context.Context, event events.KinesisEvent) error {
```

The incoming data is base64 decoded, which causes the `CloudwatchLogsRawData.Parse()` to fail. The `Parse()` method should just `continue` to enable this use case.

https://github.com/aws/aws-lambda-go/blob/0936c0677343deb60929d172f45e3707b33c7881/events/cloudwatch_logs.go#L23-L26