Check for missing fields when parsing values/instructions (#1509)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1509

Even if the type field is present and valid, the `<field>_as_<type>()` call can return null.

Check all `_as_` return values in method.cpp during load/init.

Reviewed By: kimishpatel

Differential Revision: D52451747

fbshipit-source-id: 136882982a72009edb531e8e6c5151382144cfd5