Fix a few nullability checks

lib/TweetLib.Core/Collections/TwoKeyDictionary.cs

@@ -94,7 +94,7 @@ public bool TryGetValue(K1 outerKey, K2 innerKey, out V value){
                 return innerDict.TryGetValue(innerKey, out value);
             }
             else{
-                value = default;
+                value = default!;
                 return false;
             }
         }

lib/TweetLib.Core/Data/Result.cs

@@ -16,7 +16,7 @@ public Result(T value){
         }
 
         public Result(Exception exception){
-            this.value = default;
+            this.value = default!;
             this.exception = exception ?? throw new ArgumentNullException(nameof(exception));
         }
 
@@ -25,12 +25,12 @@ public void Handle(Action<T> onSuccess, Action<Exception> onException){
                 onSuccess(value);
             }
             else{
-                onException(exception!!);
+                onException(exception!);
             }
         }
 
         public Result<R> Select<R>(Func<T, R> map){
-            return HasValue ? new Result<R>(map(value)) : new Result<R>(exception!!);
+            return HasValue ? new Result<R>(map(value)) : new Result<R>(exception!);
         }
     }
 }