[cdc-connector][mysql][hotfix] Fix NullPointerException when currentReader is null.

[loserwang1024]

In current com.ververica.cdc.connectors.mysql.source.reader.MySqlSplitReader#pollSplitRecords, if currentReader == null(for example, there is no split), NullPointerException will be thrown here:

 private MySqlRecords pollSplitRecords() throws InterruptedException {
        Iterator<SourceRecords> dataIt;
        if (currentReader == null) {
            // (1) Reads binlog split firstly and then read snapshot split
            if (binlogSplits.size() > 0) {
                // the binlog split may come from:
                // (a) the initial binlog split
                // (b) added back binlog-split in newly added table process
                MySqlSplit nextSplit = binlogSplits.poll();
                currentSplitId = nextSplit.splitId();
                currentReader = getBinlogSplitReader();
                currentReader.submitSplit(nextSplit);
            } else if (snapshotSplits.size() > 0) {
                MySqlSplit nextSplit = snapshotSplits.poll();
                currentSplitId = nextSplit.splitId();
                currentReader = getSnapshotSplitReader();
                currentReader.submitSplit(nextSplit);
            } else {
                LOG.info("No available split to read.");
            }
            dataIt = currentReader.pollSplitRecords();
          
}

[loserwang1024]

@leonardBang , @ruanhang1993 , @Shawn-Hx , CC

[loserwang1024]

@ruanhang1993 , CC

[github-actions[bot]]

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