Allow empty values on prefix headers

[Madrigal]

• fix: allow empty values on prefix headers
• Add changelog

Related smithy-go change aws/smithy-go#544

Had an issue where calling S3 with metadata with an empty value caused SDKs to not serialize the value and not send the header. This allows headers with an empty string to be serialized.

Testing

Wrote the following test program

package main

import (
	"context"
	"log"

	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/s3"
)

func main() {
	cfg, err := config.LoadDefaultConfig(context.TODO(),
		config.WithRegion("us-west-2"),
		config.WithClientLogMode(aws.LogResponseWithBody|aws.LogRequestWithBody),
	)
	if err != nil {
		log.Fatalf("error: %v", err)
	}

	svc := s3.NewFromConfig(cfg)
	_, err = svc.PutObject(context.Background(), &s3.PutObjectInput{
		Bucket: aws.String("lmadrig-test-bucket"),
		Key:    aws.String("package.json"),
		Metadata: map[string]string{
			"key1": "value1",
			"key2": "",
		},
	})
	if err != nil {
		log.Fatalf("s3 error: %v", err)
	}

Ensure the logs contain the key2 empty header

PUT /package.json?x-id=PutObject HTTP/1.1
(...)
X-Amz-Date: 20241003T212511Z
X-Amz-Meta-Key1: value1
X-Amz-Meta-Key2:

Protocol tests will be updated at smithy-lang/smithy#2415