[Bug]: ResilienceProperties works incorrectly when the value requested by a key is null

[iliar-turdushev]

Describe the bug

Methods TryGetValue and GetValue work incorrectly when the requested value is null. Specifically, TryGetValue returns false meaning it cannot find the key, and GetValue returns a value specified as a default value if the key was not found:

var key = new ResiliencePropertyKey<string?>("dummy");
var props = new ResilienceProperties();
props.Set(key, null);

props.TryGetValue(key, out var val) --> will return false, val will have null value though
props.GetValue(key, "default") --> will return a string "default", although the real value is null

Expected behavior

ResilienceProperties must return correct results even if a value being requested is null, in particular:

props.TryGetValue(key, out var val) --> must return true and val must be null
props.GetValue(key, "default") --> must return null

Actual behavior

No response

Steps to reproduce

No response

Exception(s) (if any)

No response

Polly version

8.4.1

.NET Version

net6.0 net8.0

Anything else?

No response

[iliar-turdushev]

I've opened a PR #2300 to fix this issue. Could you, please, review it? Thank you.