S3EncryptionClient builder requires region input even when endpointOverride is provided

### Problem:

`S3EncryptionClient.builder()` requires `region` as a mandatory input, even when `endpointOverride` is provided. This is a difference in behaviour from earlier version, where it was allowed to create the builder with only `endpointConfiguration` and without `region`.
Due to this, we have to pass a default region value always, resulting in ResourceNotFoundException for S3 Compatible vendors using a bucket in a different region. If no region is provided, and only endpoint is given, it works fine when using `com.amazonaws.aws`.

### Solution:

Allow `region` to be empty when `endpointOverride` is defined.

### Additional information

We are using these SDKs in Scala code
Java Version: Azul Zulu: 11.80.21
Scala Version: 2.12.19

## SDK versions
com.amazonaws.aws: 1.12.768
amazon-s3-encryption-client-java: 3.3.5
software.amazon.awssdk.s3: 2.31.67

### Sample Code
```
Using com.amazonaws.aws
AmazonS3EncryptionClientV2Builder
.standard()
.withEndpointConfiguration(
new EndpointConfiguration(endpoint, region)
)

Using software.amazon.awssdk.s3
S3EncryptionClient
.builder()
.endpointOverride(s3UriEndpoint)
.region(Region.of(s3Region)) --> Fails without passing this
```

Commencez par S3EncryptionClient.builder() et suivez la manière dont endpointOverride et la validation de region sont gérés. Comparez le comportement actuel avec l’ancien exemple endpointConfiguration ; le travail est terminé lorsqu’un builder qui ne spécifie qu’un endpoint est accepté sans exiger de region et que les utilisateurs de S3 compatible n’ont plus besoin de fournir une région par défaut potentiellement incorrecte.

Rédigé par le modèle d'indexation à partir du texte de l'issue.