[8.12](backport #38917) [filebeat][threatintel] MISP splitting fix for empty responses

[mergify]

Proposed commit message

[filebeat][threatintel] MISP splitting fix for empty responses (#)

Two fixes related to empty server responses:

- Set `response.split.ignore_empty_value` to `true` to avoid indexing
  `{response:[]}`, which is the MISP server's empty response body.
  According to the `response.split` [documentation][1], "If the split
  target is empty the parent document will be kept. If documents with
  empty splits should be dropped, the `ignore_empty_value` option should
  be set to `true`."

- Use the null-safe operator for a chained method invocation following a
  null-safe field access. All other null-safe operator usages were also
  reviewed and corrected where necessary.

[1]: https://www.elastic.co/guide/en/beats/filebeat/current/filebeat-input-httpjson.html#response-split

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

Related issues

• Closes [filebeat][threatintel][MISP] "cannot access method/field [size] from a null def reference" #38739

  ---

  This is an automatic backport of pull request [filebeat][threatintel] MISP splitting fix for empty responses #38917 done by Mergify.

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