Update HttpConnectionBase.cs (#59722)

Author: RikkiGibson

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionBase.cs

@@ -35,6 +35,7 @@ static string GetOrAddCachedValue([NotNull] ref string? cache, HeaderDescriptor
                 {
                     cache = lastValue = descriptor.GetHeaderValue(value, encoding);
                 }
+                Debug.Assert(cache is not null);
                 return lastValue;
             }
         }