```hcl
/**
 * Copyright 2021 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

locals {
  script_content = templatefile(
    "${path.module}/templates/install_spack.tpl",
    {
      ZONE        = var.zone
      PROJECT_ID  = var.project_id
      INSTALL_DIR = var.install_dir
      SPACK_URL   = var.spack_url
      SPACK_REF   = var.spack_ref
      COMPILERS   = var.compilers
      LICENSES    = var.licenses
      PACKAGES    = var.packages
      MIRRORS     = var.spack_cache_url
    }
  )
}
```

Update spack variables to be lists if not defined
user
```hcl
/**
 * Copyright 2021 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

locals {
  script_content = templatefile(
    "${path.module}/templates/install_spack.tpl",
    {
      ZONE        = var.zone
      PROJECT_ID  = var.project_id
      INSTALL_DIR = var.install_dir
      SPACK_URL   = var.spack_url
      SPACK_REF   = var.spack_ref
      COMPILERS   = var.compilers == null ? [] : var.compilers
      LICENSES    = var.licenses == null ? [] : var.licenses
      PACKAGES    = var.packages == null ? [] : var.packages
      MIRRORS     = var.spack_cache_url == null ? [] : var.spack_cache_url
    }
  )
}
```
assistant