api/s3: don't panic when final multipart version has no blocks

CompleteMultipartUpload builds the final version and then reads
`final_version.blocks.items()[0]` unconditionally. When the assembled
version ends up with an empty block list the index panics and the
process dies before it can answer the request:

    panicked at src/api/s3/multipart.rs:479:3:
    index out of bounds: the len is 0 but the index is 0

Use `.first()` and return an internal error instead, matching how the
rest of this file handles missing data. The empty-block case is an
internal consistency condition rather than a malformed request, so it
maps to an internal error rather than a 4xx.

The same panic was reported earlier in #1403, at multipart.rs:483 on
v2.2.0. That report was closed without a fix after the reporter moved
off consistency_mode = "dangerous".

Closes #1521

src/api/s3/multipart.rs

@@ -476,7 +476,13 @@ pub async fn handle_complete_multipart_upload(
 			size: total_size,
 			etag: etag.clone(),
 		},
-		final_version.blocks.items()[0].1.hash,
+		final_version
+			.blocks
+			.items()
+			.first()
+			.ok_or_internal_error("Multipart completion produced a final version with no blocks")?
+			.1
+			.hash,
 	));
 
 	let final_object = Object::new(*bucket_id, key.clone(), vec![object_version]);