Fallback to summary field if detail exists but is empty in terraform linter

[Sh1ftry]

Terraform linter incorrectly chooses empty detail field for message when both summary and detail fields exist in terraform validate output, but detail is empty. Example problematic output from terraform validate command:

{
  "format_version": "1.0",
  "valid": false,
  "error_count": 1,
  "warning_count": 0,
  "diagnostics": [
    {
      "severity": "error",
      "summary": "registry.terraform.io/hashicorp/aws: there is no package for registry.terraform.io/hashicorp/aws 4.9.0 cached in .terraform/providers",
      "detail": ""
    }
  ]
}

This PR fixes this by checking if detail is not empty before returning it.

[hsanson]

There is a linting error. Adding this linter to your ale_linters configuration would make ALE warn you about these linting issues:

 'vim': ['vint', 'ale_custom_linting_rules'],

[hsanson]

Looks good, thanks.