configs/configupgrade: Use single-line syntax for empty object exprs

configs/configupgrade/test-fixtures/valid/variable-type/input/variables.tf

@@ -4,8 +4,12 @@ variable "s" {
 
 variable "l" {
   type = "list"
+
+  default = []
 }
 
 variable "m" {
   type = "map"
+
+  default = {}
 }

configs/configupgrade/test-fixtures/valid/variable-type/want/variables.tf

@@ -4,8 +4,12 @@ variable "s" {
 
 variable "l" {
   type = list(string)
+
+  default = []
 }
 
 variable "m" {
   type = map(string)
+
+  default = {}
 }

configs/configupgrade/upgrade_expr.go

@@ -184,6 +184,10 @@ Value:
 		buf.WriteString("]")
 
 	case *hcl1ast.ObjectType:
+		if len(tv.List.Items) == 0 {
+			buf.WriteString("{}")
+			break
+		}
 		buf.WriteString("{\n")
 		for _, item := range tv.List.Items {
 			if len(item.Keys) != 1 {