Jmprieur/fix msal deserialize regression

msal/src/Microsoft.Identity.Client/Features/TokenCache/TokenCacheExtensions.cs

@@ -87,15 +87,20 @@ public static void SetBeforeWrite(this TokenCache tokencache, TokenCache.TokenCa
         /// <param name="tokenCache">Token cache to deserialize (to fill-in from the state)</param>
         /// <param name="unifiedState">Array of bytes containing serialized Msal cache data</param>
         /// <remarks>
-        /// <paramref name="unifiedState"/>Is a Json blob containing access tokens, refresh tokens, id tokens and accounts information
+        /// <paramref name="unifiedState"/>Is a Json blob containing access tokens, refresh tokens, id tokens and accounts information.
+        /// If it's <c>null</c>, then this method won't do anything (this is the same behavior as MSAL 1.x)
         /// </remarks>
         public static void Deserialize(this TokenCache tokenCache, byte[] unifiedState)
         {
-            lock (tokenCache.LockObject)
+            // Only deserialize if there is something to deserialize (like in MSAL 1.x)
+            if (unifiedState != null)
             {
-                RequestContext requestContext = new RequestContext(new MsalLogger(Guid.Empty, null));
+                lock (tokenCache.LockObject)
+                {
+                    RequestContext requestContext = new RequestContext(new MsalLogger(Guid.Empty, null));
 
-                TokenCacheSerializeHelper.DeserializeUnifiedCache(tokenCache.tokenCacheAccessor, unifiedState, requestContext);
+                    TokenCacheSerializeHelper.DeserializeUnifiedCache(tokenCache.tokenCacheAccessor, unifiedState, requestContext);
+                }
             }
         }
 

msal/tests/Test.MSAL.NET.Unit/CacheTests/TokenCacheTests.cs

@@ -780,6 +780,9 @@ public void SerializeDeserializeCacheTest()
                 ClientId = TestConstants.ClientId
             };
 
+            // Backward compatibility with MSAL 1.x, this should not throw
+            cache.Deserialize(null);
+
             MsalTokenResponse response = new MsalTokenResponse();
             response.IdToken = MockHelpers.CreateIdToken(TestConstants.UniqueId, TestConstants.DisplayableId);
             response.ClientInfo = MockHelpers.CreateClientInfo();