allocate immediately required fields

tests/required_table_default_test.fbs

@@ -0,0 +1,17 @@
+namespace RequiredTableDefaultTestNS;
+
+table Inner {
+  value: uint32;
+}
+
+// `required_inner` has no default in the wire format (tables can't have one),
+// but Pack()/Verify() both reject a buffer where it is missing - so the
+// native default constructor must never leave it null. `optional_inner` is
+// the same field type without `required`, kept alongside it to confirm the
+// fix only changes required table fields, not every table pointer field.
+table Holder {
+  required_inner: Inner (required);
+  optional_inner: Inner;
+}
+
+root_type Holder;