Refactor code in mterp and nterp to handle all cases of missing class.

This is to handle the case the type of a field is missing, and where we
only throw an exception when we assign a non-null value.

Bug: 79751666
Bug: 112676029
Bug: 176960283
Test: test.py
Change-Id: I9c233f271eea7ae4a8028fb727d5c49c7c45feac
9 files changed