[Bug][seatunnel-flink-starter]Fix NPE when starting from checkpoint.

[lightzhao]

NPE will appear when Flink is set to start from the checkpoint point.

Purpose of this pull request

Check list

• Code changed are covered with tests, or it does not need tests for reason:
• 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/incubator-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.

[lightzhao]

@Hisoka-X @TyrantLucifer PTAL, thanks.

[TyrantLucifer]

cc @ashulin PTAL.

[lightzhao]

@ashulin PTAL, thanks.

[lightzhao]

@EricJoy2048 @TyrantLucifer @Hisoka-X @ashulin PTAL.