fix null

Author: DocSvartz

src/Mapster/TypeAdapter.cs

@@ -39,6 +39,9 @@ public static TDestination Adapt<TDestination>(this object? source)
         /// <returns>Adapted destination type.</returns>
         public static TDestination Adapt<TDestination>(this object? source, TypeAdapterConfig config)
         {
+            if (source == null)
+                return default!;
+
             if (config.RuleMap.TryGetValue(new TypeTuple(source.GetType(), typeof(TDestination)), out var rule))
             {
                 if (rule.Settings.InheritsTypeTuples.Count > 0)
@@ -48,8 +51,7 @@ public static TDestination Adapt<TDestination>(this object? source, TypeAdapterC
             }
 
             // ReSharper disable once ConditionIsAlwaysTrueOrFalse
-            if (source == null)
-                return default!;
+            
             var type = source.GetType();
             var fn = config.GetDynamicMapFunction<TDestination>(type);
             return fn(source);