Resolve issue with empty string when using custom DeSerializeFn

Author: mythz

src/ServiceStack.Text/Json/JsonTypeSerializer.cs

@@ -421,7 +421,9 @@ internal static ReadOnlySpan<char> ParseJsonString(ReadOnlySpan<char> json, ref
 
         private static string UnescapeJsonString(string json, ref int index)
         {
-            return UnescapeJsonString(json.AsSpan(), ref index).ToString();
+            return json != null 
+                ? UnescapeJsonString(json.AsSpan(), ref index).ToString() 
+                : null;
         }
 
         private static ReadOnlySpan<char> UnescapeJsonString(ReadOnlySpan<char> json, ref int index) =>

tests/ServiceStack.Text.Tests/JsonTests/BasicJsonTests.cs

@@ -655,5 +655,21 @@ public void Can_deserialize_ThrowValidation_with_null_values()
             Assert.That(dto.Email, Is.EqualTo("invalidemail"));            
         }
 
+        class TestTrim
+        {
+            public string Description { get; set; }
+        }
+
+        [Test]
+        public void Can_deserialize_custom_string_deserializer()
+        {
+            JsConfig<string>.DeSerializeFn = str => str.Trim();
+            var json = "{\"description\":null}";
+            var dto = json.FromJson<TestTrim>();
+            Assert.That(dto.Description, Is.Null);
+            JsConfig<string>.DeSerializeFn = null;
+        }
+
+        
     }
 }