[viewchange] simplify GetPreparedBlock function

Signed-off-by: Leo Chen <leo@harmony.one>

consensus/view_change_construct.go

@@ -113,18 +113,19 @@ func (vc *viewChange) GetPreparedBlock(fbftlog *FBFTLog, hash [32]byte) ([]byte,
 	vc.vcLock.RLock()
 	defer vc.vcLock.RUnlock()
 
-	if !vc.isM1PayloadEmpty() {
-		block := fbftlog.GetBlockByHash(hash)
-		if block != nil {
+	if vc.isM1PayloadEmpty() {
+		return nil, nil
+	}
+
+	if block := fbftlog.GetBlockByHash(hash); block != nil {
 		encodedBlock, err := rlp.EncodeToBytes(block)
 		if err != nil || len(encodedBlock) == 0 {
-				vc.getLogger().Err(err).Msg("[GetPreparedBlock] Failed encoding prepared block")
-				return vc.m1Payload, nil
+			return nil, nil
 		}
 		return vc.m1Payload, encodedBlock
 	}
-	}
-	return vc.m1Payload, nil
+
+	return nil, nil
 }
 
 // GetM2Bitmap returns the nilBitmap as M2Bitmap