Update ResilienceProperties to correctly handle null values (#2300)

Adds logic handling a case when the value being requested is null.

src/Polly.Core/ResilienceProperties.cs

@@ -20,10 +20,24 @@ public sealed class ResilienceProperties
     /// <returns>True, if a property was retrieved.</returns>
     public bool TryGetValue<TValue>(ResiliencePropertyKey<TValue> key, [MaybeNullWhen(false)] out TValue value)
     {
-        if (Options.TryGetValue(key.Key, out object? val) && val is TValue typedValue)
+        if (Options.TryGetValue(key.Key, out object? val))
         {
-            value = typedValue;
-            return true;
+            if (val is TValue typedValue)
+            {
+                value = typedValue;
+                return true;
+            }
+            else if (val is null)
+            {
+                // We have to use null-forgiving operator "!" here to suppress a null-state analysis warning.
+                // The reason is the following. The output type "TValue" doesn't have any type constraints as
+                // "notnull", "class" or "struct", therefore the analyzer considers "TValue" as non-nullable
+                // and warns us that we're assigning "null" to it. But that's not correct, because "TValue"
+                // could be a nullable type, e.g. "string?", and assigning "null" to it is correct. Therefore
+                // it is reasonable to use "!" here to suppress the warning.
+                value = default!;
+                return true;
+            }
         }
 
         value = default;

test/Polly.Core.Tests/ResiliencePropertiesTests.cs

@@ -14,6 +14,18 @@ public void TryGetValue_Ok()
         val.Should().Be(12345);
     }
 
+    [Fact]
+    public void TryGetValue_ValueIsNull_Ok()
+    {
+        var key = new ResiliencePropertyKey<string?>("dummy");
+        var props = new ResilienceProperties();
+
+        props.Set(key, null);
+
+        props.TryGetValue(key, out var val).Should().Be(true);
+        val.Should().Be(null);
+    }
+
     [Fact]
     public void TryGetValue_NotFound_Ok()
     {
@@ -34,6 +46,17 @@ public void GetValue_Ok()
         props.GetValue(key, default).Should().Be(12345);
     }
 
+    [Fact]
+    public void GetValue_ValueIsNull_Ok()
+    {
+        var key = new ResiliencePropertyKey<string?>("dummy");
+        var props = new ResilienceProperties();
+
+        props.Set(key, null);
+
+        props.GetValue(key, "default").Should().Be(null);
+    }
+
     [Fact]
     public void GetValue_NotFound_EnsureDefault()
     {