Fix bug with unquoting null from JSON-encoded HCL

Fixes hashicorp/terraform#6774.

decoder_test.go

@@ -109,6 +109,19 @@ func TestDecode_interface(t *testing.T) {
 			false,
 			map[string]interface{}{"foo": "bar\nbaz"},
 		},
+		{
+			"null_strings.json",
+			false,
+			map[string]interface{}{
+				"module": []map[string]interface{}{
+					map[string]interface{}{
+						"app": []map[string]interface{}{
+							map[string]interface{}{"foo": ""},
+						},
+					},
+				},
+			},
+		},
 		{
 			"scientific.json",
 			false,

hcl/token/token.go

@@ -152,6 +152,11 @@ func (t Token) Value() interface{} {
 			f = strconv.Unquote
 		}
 
+		// This case occurs if json null is used
+		if t.Text == "" {
+			return ""
+		}
+
 		v, err := f(t.Text)
 		if err != nil {
 			panic(fmt.Sprintf("unquote %s err: %s", t.Text, err))

test-fixtures/null_strings.json

@@ -0,0 +1,7 @@
+{
+  "module": {
+    "app": {
+      "foo": null
+    }
+  }
+}