Event should show as non-nullable where not assignable or invokable

[sharwell]

Version Used: 16.6 Preview 3

Steps to Reproduce:

#nullable enable

using System;

class First
{
    public event EventHandler? Handler;
}

class Second
{
    public void Method(First first)
    {
        first.Handler += delegate { }; // ← hover over 'Handler'
    }
}

Expected Behavior:

EventHandler First.Handler

Actual Behavior:

EventHandler? First.Handler

[jcouv]

We have implemented no nullability checking for events yet. That is tracked by #29839