tls: do not break custom fields when enabling JA4 (7.x)

[satta]

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• I have read the contributing guide lines at
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Changes (if applicable):

• I have created a ticket at
  https://redmine.openinfosecfoundation.org/projects/suricata/issues

Link to ticket: https://redmine.openinfosecfoundation.org/issues/7286

Describe changes:

• Ensure tls_ctx->fields is not overwritten when selecting JA4 log output.

[jufajardini]

Thanks! Creating a test for this also exposed the fact that our json schema is missing two tls custom fields...