[Fix][Connector-V2] Fix cdc use default value when value is null

[Hisoka-X]

Purpose of this pull request

Fix cdc use default value when value is null. We should not use default value when the value is null.

Does this PR introduce any user-facing change?

Yes, bug fixed.

How was this patch tested?

add new test.

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If you are contributing the connector code, please check that the following files are updated:
  1. Update plugin-mapping.properties and add new connector information in it
  2. Update the pom file of seatunnel-dist
  3. Add ci label in label-scope-conf
  4. Add e2e testcase in seatunnel-e2e
  5. Update connector plugin_config
• Update the release-note.

[liugddx]

LGTM