[Fix] Fix drift in `databricks_cluster` when `is_single_node` is set to `true`

[ashenm]

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Changes

Fixes #4360

Tests

• [x] make test run locally
• [ ] relevant change in docs/ folder
• [ ] covered with integration tests in internal/acceptance
• [ ] using Go SDK
• [ ] using TF Plugin Framework

In addition to the unit tests, deployed provider locally with following resource definition and verified diff

resource "databricks_cluster" "single_node" {
  spark_version      = "15.4.x-scala2.12"
  single_user_name   = "[email protected]"
  runtime_engine     = "STANDARD"
  node_type_id       = "r5.large"
  kind               = "CLASSIC_PREVIEW"
  is_single_node     = true
  data_security_mode = "SINGLE_USER"
  cluster_name       = upper("demo-single-node")

  custom_tags = {
    "owner" = "ashenm"
  }
  autotermination_minutes = 10
}