fix: disambiguate variable defaults with "empty" values from undefined

- the default is wrapped in a struct, so that the wrapper being nil
  can indicate that `default` was undefined. if a default was defined,
  the Value field is serialized

tfconfig/load_hcl.go

@@ -148,7 +148,7 @@ func loadModule(dir string) (*Module, Diagnostics) {
 							// guaranteed valid by ctyjson.Marshal.
 							panic(fmt.Errorf("failed to re-parse default value from JSON: %s", err))
 						}
-						v.Default = def
+						v.Default = &Default{Value: def}
 					}
 				}
 

tfconfig/load_legacy.go

@@ -101,7 +101,7 @@ func loadModuleLegacyHCL(dir string) (*Module, Diagnostics) {
 					Name:        name,
 					Type:        block.Type,
 					Description: block.Description,
-					Default:     block.Default,
+					Default:     &Default{Value: block.Default},
 					Pos:         sourcePosLegacyHCL(item.Pos(), filename),
 				}
 				if _, exists := mod.Variables[name]; exists {

tfconfig/variable.go

@@ -1,5 +1,13 @@
 package tfconfig
 
+import (
+	"encoding/json"
+)
+
+type Default struct {
+	Value interface{}
+}
+
 // Variable represents a single variable from a Terraform module.
 type Variable struct {
 	Name        string `json:"name"`
@@ -10,7 +18,11 @@ type Variable struct {
 	// the native Go type system. The conversion from the value given in
 	// configuration may be slightly lossy. Only values that can be
 	// serialized by json.Marshal will be included here.
-	Default interface{} `json:"default,omitempty"`
+	Default *Default `json:"default,omitempty"`
 
 	Pos SourcePos `json:"pos"`
 }
+
+func (d *Default) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&d.Value)
+}