fix: listing SSE encrypted multipart objects (#18786)

GetActualSize() was heavily relying on o.Parts()
to be non-empty to figure out if the object is multipart or not, 
However, we have many indicators of whether an object is multipart 
or not.

Blindly assuming that o.Parts == nil is not a multipart, is an 
incorrect expectation instead, multipart must be obtained via

- Stored metadata value indicating this is a multipart encrypted object.

- Rely on <meta>-actual-size metadata to get the object's actual size.
  This value is preserved for additional reasons such as these.

- ETag != 32 length

cmd/erasure-healing.go

@@ -334,7 +334,7 @@ func (er *erasureObjects) healObject(ctx context.Context, bucket string, object
 			erasure.ShardFileSize(latestMeta.Parts[0].ActualSize) < smallFileThreshold)
 	}
 
-	result.ObjectSize, err = latestMeta.GetActualSize()
+	result.ObjectSize, err = latestMeta.ToObjectInfo(bucket, object, true).GetActualSize()
 	if err != nil {
 		return result, err
 	}