fix normalizeNullValues when apply resource to accept an empty list

and doesn't use a null src otherwise a drift is generate when refresh resource
Fixes #766

Author: jeremmfr

helper/schema/grpc_provider.go

@@ -1261,13 +1261,17 @@ func normalizeNullValues(dst, src cty.Value, apply bool) cty.Value {
 	}
 
 	// Handle null/empty changes for collections during apply.
-	// A change between null and empty values prefers src to make sure the state
+	// A change from emtpy to null values prefers src to make sure the state
 	// is consistent between plan and apply.
+	// A change from null to empty values prefers dst to not generate drifts of
+	// values when refresh.
 	if ty.IsCollectionType() && apply {
 		dstEmpty := !dst.IsNull() && dst.IsKnown() && dst.LengthInt() == 0
 		srcEmpty := !src.IsNull() && src.IsKnown() && src.LengthInt() == 0
 
-		if (src.IsNull() && dstEmpty) || (srcEmpty && dst.IsNull()) {
+		if src.IsNull() && dstEmpty {
+			return dst
+		} else if srcEmpty && dst.IsNull() {
 			return src
 		}
 	}

helper/schema/grpc_provider_test.go

@@ -1369,7 +1369,7 @@ func TestNormalizeNullValues(t *testing.T) {
 				"network_interface": cty.ListVal([]cty.Value{
 					cty.ObjectVal(map[string]cty.Value{
 						"network_ip":    cty.StringVal("10.128.0.64"),
-						"access_config": cty.NullVal(cty.List(cty.Object(map[string]cty.Type{"public_ptr_domain_name": cty.String, "nat_ip": cty.String}))),
+						"access_config": cty.ListValEmpty(cty.Object(map[string]cty.Type{"public_ptr_domain_name": cty.String, "nat_ip": cty.String})),
 						"address":       cty.StringVal("address"),
 						"name":          cty.StringVal("nic0"),
 					}),
@@ -1529,9 +1529,9 @@ func TestNormalizeNullValues(t *testing.T) {
 				})),
 			}),
 			Expect: cty.ObjectVal(map[string]cty.Value{
-				"set": cty.NullVal(cty.Set(cty.Object(map[string]cty.Type{
+				"set": cty.SetValEmpty(cty.Object(map[string]cty.Type{
 					"list": cty.List(cty.String),
-				}))),
+				})),
 			}),
 			Apply: true,
 		},