[oracle-cdc] Fix read one record in the incremental snapshot

[e-mhui]

Fix bug: #1886

In the snapshot, record.key() is null, so it will overwrite the previous record. So we can to add record without primary key into list.

  if (!reachChangeLogStart) {
      if (record.key() == null) {
          snapshotRecordsWithoutKey.add(record);
      } else {
          outputBuffer.put((Struct) record.key(), record);
      }
  }

[ruanhang1993]

Maybe this is a new feature that make oracle cdc support the tables without primary keys.

[yuxiqian]

Thanks @e-mhui for the great work! Before this PR could be merged, could you please rebase it with latest master branch?

cc @ruanhang1993

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