Fix crash when variables block is undefined

This change fixes a crash that occurs when trying to add undeclared variables into an empty
variables map. The variables block when defined will always be non-nil but in the case
where variables are declared and initialized from a legacy JSON variables file the map
must be initialized first.

command/hcl2_upgrade.go

@@ -870,6 +870,9 @@ func (p *VariableParser) Parse(tpl *template.Template) error {
 		p.localsOut = []byte{}
 	}
 
+	if len(tpl.Variables) == 0 {
+		tpl.Variables = make(map[string]*template.Variable)
+	}
 	// JSON supports variable declaration via var-files.
 	// User variables that might be defined in a var-file
 	// but not in the actual JSON template should be accounted for.

command/hcl2_upgrade_test.go

@@ -34,6 +34,7 @@ func Test_hcl2_upgrade(t *testing.T) {
 		{folder: "variables-with-variables", flags: []string{}},
 		{folder: "complete-variables-with-template-engine", flags: []string{}},
 		{folder: "undeclared-variables", flags: []string{}, exitCode: 0},
+		{folder: "varfile-with-no-variables-block", flags: []string{}, exitCode: 0},
 	}
 
 	for _, tc := range tc {

command/test-fixtures/hcl2_upgrade/varfile-with-no-variables-block/expected.pkr.hcl

@@ -0,0 +1,33 @@
+
+variable "ssh_host" {
+  type = string
+}
+
+variable "ssh_password" {
+  type = string
+}
+
+variable "ssh_username" {
+  type = string
+}
+
+variable "version_tag" {
+  type = string
+}
+
+source "null" "autogenerated_1" {
+  communicator = "ssh"
+  ssh_host     = "${var.ssh_host}"
+  ssh_password = "${var.ssh_password}"
+  ssh_username = "${var.ssh_username}"
+}
+
+build {
+  sources = ["source.null.autogenerated_1"]
+
+  provisioner "shell-local" {
+    inline       = ["echo ${var.version_tag} > provision.txt"]
+    pause_before = "20s"
+  }
+
+}

command/test-fixtures/hcl2_upgrade/varfile-with-no-variables-block/input.json

@@ -0,0 +1,18 @@
+{
+  "builders": [
+    {
+      "type": "null",
+      "communicator": "ssh",
+      "ssh_host": "{{ user `ssh_host` }}", 
+      "ssh_password": "{{ user `ssh_password` }}",
+      "ssh_username": "{{ user `ssh_username` }}"
+    }
+  ],
+  "provisioners": [
+    {
+        "pause_before": "20s",
+        "type": "shell-local",
+        "inline": [ "echo {{ user `version_tag` }} > provision.txt" ]
+    }
+  ]
+}

command/test-fixtures/hcl2_upgrade/varfile-with-no-variables-block/variables.json

@@ -0,0 +1,6 @@
+{
+  "version_tag":      "1.0.0",
+  "ssh_host":         "localhost",
+  "ssh_username":     "packer",
+  "ssh_password":     "packer"
+}