refactor: move complex conditional to a method

The `f.SQLDefault == ""` part seems redundant before addReturningField,
as RETURNING makes sense for a field that was given its default value.

internal/dbtest/query_test.go

@@ -331,8 +331,8 @@ func TestQuery(t *testing.T) {
 				Where("model.id = _data.id")
 		},
 		func(db *bun.DB) schema.QueryAppender {
-			// "nullzero" marshalls zero values as DEFAULT or NULL (if DEFAULT placeholder is not supported)
-			// DB drivers that support DEFAULT placeholder resolve it to NULL for columns that do not have a DEFAULT value.
+			// "nullzero" marshals zero values as DEFAULT or NULL (if DEFAULT placeholder is not supported)
+			// DB drivers which support DEFAULT placeholder resolve it to NULL for columns that do not have a DEFAULT value.
 			type Model struct {
 				Int  int64     `bun:",nullzero"`
 				Uint uint64    `bun:",nullzero"`
@@ -1019,7 +1019,7 @@ func TestQuery(t *testing.T) {
 			}).Column("is_active").WherePK()
 		},
 		func(db *bun.DB) schema.QueryAppender {
-			// "default" marshalls zero values as DEFAULT or specified default value
+			// "default" marshals zero values as DEFAULT or the specified default value
 			type Model struct {
 				Int      int64     `bun:",default:42"`
 				Uint     uint64    `bun:",default:42"`