[Hotfix][Core] Fix the NullPointException for the json config of the job without pluginname

[dailai]

Purpose of this pull request

When we use the json config of the job define that without pluginname, this case will throw NullPointerException.

Like this:
{ "env": { "parallelism": 4, "job.mode": "BATCH" }, "source": [ { "result_table_name": "fake", "row.num": 100, "schema": { "fields": { "name": "string", "age": "int", "card": "int" } } } ], "transform": [ ], "sink": [ { "source_table_name": "fake1" } ] }

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.

[Hisoka-X]

I think we should check flink and spark job too.

[dailai]

I think we should check flink and spark job too.

Yes,

I think we should check flink and spark job too.

That's right. I haven't committed my code yet

[Hisoka-X]

LGTM. Thanks @dailai !