Remove some `= null!`s from corelib (#36175)

Remove some `= null!`s from corelib (#36175)

* Remove some `= null!`s from corelib

In some cases, this was achieved by:
- Using [MemberNotNull] when it could be used to enable the compiler to appropriately follow the initialization
- Making the field nullable and using `!`s at usage sites in cases where the `= null!` wasn't appropriate because the field could actually be null
- Making the field nullable and adding null checks sparingly
- Suppressing warnings in cases where fields weren't accessed in managed code
- Removing stale cases where it wasn't necessary anymore anyway
- Removing unnecessary empty internal/private ctors

* Address PR feedback

* Disable test on mono