Fix panic on upgrading variables json file

command/hcl2_upgrade.go

@@ -896,6 +896,10 @@ type BuildParser struct {
 }
 
 func (p *BuildParser) Parse(tpl *template.Template) error {
+	if len(p.Builders) == 0 {
+		return nil
+	}
+
 	buildContent := hclwrite.NewEmptyFile()
 	buildBody := buildContent.Body()
 	if tpl.Description != "" {

command/hcl2_upgrade_test.go

@@ -26,6 +26,7 @@ func Test_hcl2_upgrade(t *testing.T) {
 		{folder: "source-name", flags: []string{"-with-annotations"}},
 		{folder: "error-cleanup-provisioner", flags: []string{"-with-annotations"}},
 		{folder: "aws-access-config", flags: []string{}},
+		{folder: "variables-only", flags: []string{}},
 	}
 
 	for _, tc := range tc {

command/test-fixtures/hcl2_upgrade/variables-only/expected.pkr.hcl

@@ -0,0 +1,45 @@
+
+variable "aws_access_key" {
+  type      = string
+  default   = ""
+  sensitive = true
+}
+
+variable "aws_region" {
+  type = string
+}
+
+variable "aws_secondary_region" {
+  type    = string
+  default = "${env("AWS_DEFAULT_REGION")}"
+}
+
+variable "aws_secret_key" {
+  type      = string
+  default   = ""
+  sensitive = true
+}
+
+variable "secret_account" {
+  type      = string
+  default   = "🤷"
+  sensitive = true
+}
+
+data "amazon-secretsmanager" "autogenerated_1" {
+  name = "sample/app/password"
+}
+
+data "amazon-secretsmanager" "autogenerated_2" {
+  key  = "api_key"
+  name = "sample/app/passwords"
+}
+
+local "password" {
+  sensitive  = true
+  expression = "${data.amazon-secretsmanager.autogenerated_1.value}"
+}
+
+locals {
+  password_key = "MY_KEY_${data.amazon-secretsmanager.autogenerated_2.value}"
+}

command/test-fixtures/hcl2_upgrade/variables-only/input.json

@@ -0,0 +1,18 @@
+{
+    "variables": {
+        "secret_account": "🤷",
+        "aws_region": null,
+        "aws_secondary_region": "{{ env `AWS_DEFAULT_REGION` }}",
+        "aws_secret_key": "",
+        "aws_access_key": "",
+        "password": "{{ aws_secretsmanager `sample/app/password` }}",
+        "password_key": "MY_KEY_{{ aws_secretsmanager `sample/app/passwords` `api_key` }}"
+    },
+    "sensitive-variables": [
+        "aws_secret_key",
+        "aws_access_key",
+        "secret_account",
+        "potato",
+        "password"
+    ]
+}