[Nullable analysis] Assigning default to struct variable with non-nulable refs should produce nullability warning

[petrroll]

Version Used:
Latest

Steps to Reproduce:

1. Create a struct MyStruct with non-nullable field / property object MyProperty
2. Create a variable MyVar of type MyStruct
3. Assign default to MyVar

A minimal repro, with source-code provided, is ideal:
https://sharplab.io/#v2:C4LgTgrgdgNAJiA1AHwAICYCMBYAUKgZgAJVMA2E9IgYSIG88inKqHdmOiA3AQzCIBGRALxE4AUwBmPCABtgACgBCASgDcjTkwEA6ALIBPAApgA9gAdxYYAZ0AVUwGVgYAJZQA5gvWamAX18iPADcPDBxHjhTKFkDInCAY1MwOCIAZxcIBOAiJQVTAQArcWyiQxMLKxt1IA=

Expected Behavior:
Some sort of warning happens.

Actual Behavior:
it's all good, acessing MyProperty trows NRE.

Notes
I understand that by definition nullability analysis has holes. One could e.g. create an array of such structs (or generally initialization of arrays is a problem), we could be in generic type situation, etc.

And also understand that plugging even this generally would probably require adding a notion of non-default-initialized struct and flow that through analysis or some way to disallow usage of default generated constructor (maybe warn if there's one autogenerated on struct with non-nullable fields?) to fix situations s.a. with initialization of field on class.

But plugging at least the case where we explicitly pass default to such variable should be possible .