[bugfix][mongodb] Fixed unsupported exception caused by bsonNull

[ic4y]

Purpose of this pull request

Does this PR introduce any user-facing change?

How was this patch tested?

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 change log that in connector document. For more details you can refer to connector-v2
  2. Update plugin-mapping.properties and add new connector information in it
  3. Update the pom file of seatunnel-dist
• Update the release-note.

[ic4y]

@Carl-Zhou-CN @MonsterChenzhuo PTAL

[Carl-Zhou-CN]

LGTM

[Hisoka-X]

Could you add some comment for this PR?