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/encryption-v1.go

@@ -209,9 +209,6 @@ func DecryptETags(ctx context.Context, k kms.KMS, objects []ObjectInfo) error {
 // uploaded by the user using multipart mechanism:
 // initiate new multipart, upload part, complete upload
 func (o *ObjectInfo) isMultipart() bool {
-	if len(o.Parts) == 0 {
-		return false
-	}
 	_, encrypted := crypto.IsEncrypted(o.UserDefined)
 	if encrypted {
 		if !crypto.IsMultiPart(o.UserDefined) {
@@ -228,7 +225,7 @@ func (o *ObjectInfo) isMultipart() bool {
 	// Further check if this object is uploaded using multipart mechanism
 	// by the user and it is not about Erasure internally splitting the
 	// object into parts in PutObject()
-	return !(o.backendType == BackendErasure && len(o.ETag) == 32)
+	return len(o.ETag) != 32
 }
 
 // ParseSSECopyCustomerRequest parses the SSE-C header fields of the provided request.