TypeDescriptor.GetProperties should not have nullable items

[virzak]

Version Used:
VS 16.4, .Net Core 3.1

Steps to Reproduce:

var allProperties =
    TypeDescriptor.GetProperties(typeof(T));

var pds = new List<PropertyDescriptor>();

foreach (PropertyDescriptor prop in allProperties)
{
}

Expected Behavior:
No warnings

Actual Behavior:
CS8606 // Possible null reference assignment to iteration variable

---

How can any PropertyDescriptor be null?

[jinujoseph]

@jaredpar do you have access to move to runtime repo

[ericstj]

We haven't annotated TypeConverter assembly yet. See #2339