Fix missing region error when using Firelens external config.

GetBucketLocation returns null for bucket in us-east-1. Return us-east-1 instead of null in that case.

agent/s3/factory/factory.go

@@ -28,7 +28,8 @@ import (
 )
 
 const (
-	roundtripTimeout = 5 * time.Second
+	bucketLocationDefault = "us-east-1"
+	roundtripTimeout      = 5 * time.Second
 )
 
 type S3ClientCreator interface {
@@ -69,6 +70,9 @@ func getRegionFromBucket(svc *s3.S3, bucket string) (string, error) {
 	if err != nil {
 		return "", err
 	}
+	if result.LocationConstraint == nil { // GetBucketLocation returns nil for bucket in us-east-1.
+		return bucketLocationDefault, nil
+	}
 
 	return aws.StringValue(result.LocationConstraint), nil
 }