Expand fields in decode_json_fields if target is set

[kvch]

What does this PR do?

This PR applies the settings in expand_keys to the event even if the target is not empty.

Why is it important?

Previously, if target was set to anything besides "", expand_keys did not work. So you could not expand the keys of the parsed JSON if you wanted to put it under a custom field.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Config:

filebeat.inputs:
- type: filestream
  id: my-filestream-id
  paths:
    - test.log
  processors:
    - decode_json_fields:
      target: "my_target"
      expand_fields: true
      fields: ["message"]
output.console:
  enabled: true

Example input file:

{"my.field": "interesting value"}

Check if the keys are expanded in the published event:

{
    "message": "{\"my.field\": \"interesting value\"}",
    "my_target": {
        "my": {
          "field": "interesting value"
        }
    }
}

Related issues

Closes #31712

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• Start Time: 2022-06-21T13:13:59.676+0000

• Duration: 80 min 13 sec

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Skipped	1937
Total	24217

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