While I was working on #18703, I discovered a bad behavior when people don't provide AWS key/secret as part as their `elasticsearch.yml` but rely on SysProps or env. variables...
In [`InternalAwsS3Service#getClient(...)`](d4366f8493/plugins/repository-s3/src/main/java/org/elasticsearch/cloud/aws/InternalAwsS3Service.java (L76-L141)), we have:
```java
Tuple<String, String> clientDescriptor = new Tuple<>(endpoint, account);
AmazonS3Client client = clients.get(clientDescriptor);
```
But if people don't provide credentials, `account` is `null`.
Even if it actually could work, I think that we should use the `AWSCredentialsProvider` we create later on and extract from it the `account` (AWS KEY actually) and then use it as the second value of the tuple.
Closes#19557.
f8e0557be5