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/object-api-utils.go

@@ -533,6 +533,14 @@ func (o ObjectInfo) GetActualSize() (int64, error) {
 		return size, nil
 	}
 	if _, ok := crypto.IsEncrypted(o.UserDefined); ok {
+		sizeStr, ok := o.UserDefined[ReservedMetadataPrefix+"actual-size"]
+		if ok {
+			size, err := strconv.ParseInt(sizeStr, 10, 64)
+			if err != nil {
+				return -1, errObjectTampered
+			}
+			return size, nil
+		}
 		return o.DecryptedSize()
 	}
 
@@ -632,16 +640,14 @@ func getCompressedOffsets(oi ObjectInfo, offset int64, decrypt func([]byte) ([]b
 	var skipLength int64
 	var cumulativeActualSize int64
 	var firstPartIdx int
-	if len(oi.Parts) > 0 {
-		for i, part := range oi.Parts {
-			cumulativeActualSize += part.ActualSize
-			if cumulativeActualSize <= offset {
-				compressedOffset += part.Size
-			} else {
-				firstPartIdx = i
-				skipLength = cumulativeActualSize - part.ActualSize
-				break
-			}
+	for i, part := range oi.Parts {
+		cumulativeActualSize += part.ActualSize
+		if cumulativeActualSize <= offset {
+			compressedOffset += part.Size
+		} else {
+			firstPartIdx = i
+			skipLength = cumulativeActualSize - part.ActualSize
+			break
 		}
 	}
 	partSkip = offset - skipLength