[Minor][FLINK-39168][cdc-connector-mysql] Cache split.splitId() result in a local variable to avoid calling the method twice per iteration: (apache#4288)

Co-authored-by: Thorne <syyfffy@163.com>

Author: ThorneANN

flink-cdc-connect/flink-cdc-source-connectors/flink-connector-mysql-cdc/src/main/java/org/apache/flink/cdc/connectors/mysql/source/assigners/MySqlSnapshotSplitAssigner.java

@@ -400,11 +400,12 @@ public List<FinishedSnapshotSplitInfo> getFinishedSplitInfos() {
                 new ArrayList<>(assignedSplits.values());
         List<FinishedSnapshotSplitInfo> finishedSnapshotSplitInfos = new ArrayList<>();
         for (MySqlSchemalessSnapshotSplit split : assignedSnapshotSplit) {
-            BinlogOffset binlogOffset = splitFinishedOffsets.get(split.splitId());
+            String splitId = split.splitId();
+            BinlogOffset binlogOffset = splitFinishedOffsets.get(splitId);
             finishedSnapshotSplitInfos.add(
                     new FinishedSnapshotSplitInfo(
                             split.getTableId(),
-                            split.splitId(),
+                            splitId,
                             split.getSplitStart(),
                             split.getSplitEnd(),
                             binlogOffset));