handle null NestingSingle values

Null NestingSingle attributes were not being handled in ProposedNew

internal/plans/objchange/objchange.go

@@ -311,8 +311,11 @@ func proposedNewNestedType(schema *configschema.Object, prior, config cty.Value)
 	var newV cty.Value
 	switch schema.Nesting {
 	case configschema.NestingSingle:
-		newAttrs := proposedNewAttributes(schema.Attributes, prior, config)
-		newV = cty.ObjectVal(newAttrs)
+		if !config.IsNull() {
+			newV = cty.ObjectVal(proposedNewAttributes(schema.Attributes, prior, config))
+		} else {
+			newV = cty.NullVal(config.Type())
+		}
 
 	case configschema.NestingList:
 		// Nested blocks are correlated by index.

internal/plans/objchange/objchange_test.go

@@ -1433,7 +1433,7 @@ func TestProposedNew(t *testing.T) {
 				}),
 			}),
 			cty.ObjectVal(map[string]cty.Value{
-				"single": cty.ObjectVal(map[string]cty.Value{"bar": cty.NullVal(cty.String)}),
+				"single": cty.NullVal(cty.Object(map[string]cty.Type{"bar": cty.String})),
 				"list":   cty.NullVal(cty.List(cty.Object(map[string]cty.Type{"bar": cty.String}))),
 				"map":    cty.NullVal(cty.Map(cty.Object(map[string]cty.Type{"bar": cty.String}))),
 				"set":    cty.NullVal(cty.Set(cty.Object(map[string]cty.Type{"bar": cty.String}))),
@@ -1447,7 +1447,7 @@ func TestProposedNew(t *testing.T) {
 				}))),
 			}),
 			cty.ObjectVal(map[string]cty.Value{
-				"single": cty.ObjectVal(map[string]cty.Value{"bar": cty.NullVal(cty.String)}),
+				"single": cty.NullVal(cty.Object(map[string]cty.Type{"bar": cty.String})),
 				"list":   cty.NullVal(cty.List(cty.Object(map[string]cty.Type{"bar": cty.String}))),
 				"map":    cty.NullVal(cty.Map(cty.Object(map[string]cty.Type{"bar": cty.String}))),
 				"set":    cty.NullVal(cty.Set(cty.Object(map[string]cty.Type{"bar": cty.String}))),