Ignore empty fields in API response

[tobio]

Fixes #836

Introduces some new type helpers for the case where we want to treat an empty string as null.

To reproduce the original bug:

1. Create a fleet output via Terraform, this resource is enough:

resource "elasticstack_fleet_output" "elasticsearch_output" {
  name                 = "elasticsearch-output"
  type                 = "elasticsearch"
  config_yaml          = yamlencode({})
  default_integrations = true
  default_monitoring   = true
  hosts = [
    "https://tb-es-http.elastic-system.svc:9200",
    "https://elasticsearch.acme.test.com:443" # added
  ]
}

2. In the Fleet settings UI, update the name of the output and save it
3. terraform apply

The current provider version will error, this PR will not. It's hard to create an acceptance test around this behaviour since it requires editing the output via the UI (I suppose I could have a test that modified the output directly via the API).

[Copilot]

Pull request overview

This PR fixes a bug where empty string fields in API responses were not being properly handled, causing Terraform apply operations to fail after manual UI updates. The fix introduces new type utility functions to treat empty strings as null values, ensuring consistent state management between Terraform and the API.

Key changes:

• New utility functions NonEmptyStringValue and NonEmptyStringPointerValue to convert empty strings to null
• Updated SSL configuration handling to return null objects when all fields are empty
• Added validators to ensure required SSL fields are non-empty

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/utils/typeutils/string.go	Added utility functions to convert empty strings to null values
internal/fleet/output/schema.go	Added validators to ensure SSL certificate fields are non-empty
internal/fleet/output/models_ssl_test.go	Added test cases for empty SSL field handling
internal/fleet/output/models_ssl.go	Updated SSL model conversion to use new utilities and handle empty fields
internal/fleet/output/models_logstash.go	Applied empty string handling to CaTrustedFingerprint field
internal/fleet/output/models_kafka.go	Applied empty string handling to CaTrustedFingerprint field
internal/fleet/output/models_elasticsearch.go	Applied empty string handling to CaTrustedFingerprint field
internal/elasticsearch/ml/anomaly_detection_job/models_tf.go	Replaced verbose empty string checks with new utility functions throughout ML model conversions
internal/elasticsearch/index/alias/models.go	Applied empty string handling to routing fields