Test platform crashes when there's null value in Payload messages

[nigurr]

When tests are being discovered/executed if the adapter sends the value as "null" in the payload, platform fails to deserialize the payload causing it crash.

For example:

   {
      "Key": {
        "Id": "VCTestDiscoverer.TestCategory",
        "Label": "TestCategory",
        "Category": "",
        "Description": "",
        "Attributes": 1,
        "ValueType": "System.String[]"
      },
      "Value": null
    },

TpTrace Verbose: 0 : 4936, 7, 2018/05/31, 17:21:44.662, 12516221489, vstest.console.exe, TestRequestSender: GetAbortErrorMessage: Exception: Newtonsoft.Json.JsonSerializationException: Error setting value to 'StoreKeyValuePairs' on 'Microsoft.VisualStudio.TestPlatform.ObjectModel.TestCase'. ---> System.NotSupportedException: CustomStringArrayConverter cannot convert from (null).
   at System.ComponentModel.TypeConverter.GetConvertFromException(Object value)
   at System.ComponentModel.TypeConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at Microsoft.VisualStudio.TestPlatform.ObjectModel.CustomStringArrayConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at Microsoft.VisualStudio.TestPlatform.ObjectModel.TestObject.ConvertPropertyFrom[T](TestProperty property, CultureInfo culture, Object value)
   at Microsoft.VisualStudio.TestPlatform.ObjectModel.TestObject.SetPropertyValue[T](TestProperty property, T value, CultureInfo culture)
   at Microsoft.VisualStudio.TestPlatform.ObjectModel.TestObject.set_StoreKeyValuePairs(List`1 value)
   at SetStoreKeyValuePairs(Object , Object )
   at Newtonsoft.Json.Serialization.DynamicValueProvider.SetValue(Object target, Object value)
   --- End of inner exception stack trace ---
   at Newtonsoft.Json.Serialization.DynamicValueProvider.SetValue(Object target, Object value)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.SetPropertyValue(JsonProperty property, JsonConverter propertyConverter, JsonContainerContract containerContract, JsonProperty containerProperty, JsonReader reader, Object target)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.PopulateObject(Object newObject, JsonReader reader, JsonObjectContract contract, JsonProperty member, String id)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateObject(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.PopulateList(IList list, JsonReader reader, JsonArrayContract contract, JsonProperty containerProperty, String id)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateList(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, Object existingValue, String id)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at Newtonsoft.Json.Linq.JToken.ToObject(Type objectType, JsonSerializer jsonSerializer)
   at Newtonsoft.Json.Linq.JToken.ToObject[T](JsonSerializer jsonSerializer)
   at Microsoft.VisualStudio.TestPlatform.CommunicationUtilities.JsonDataSerializer.DeserializePayload[T](Message message)
   at Microsoft.VisualStudio.TestPlatform.CommunicationUtilities.TestRequestSender.OnDiscoveryMessageReceived(ITestDiscoveryEventsHandler2 discoveryEventsHandler, MessageReceivedEventArgs args)
TpTrace Error: 0 : 4936, 7, 2018/05/31, 17:21:44.662, 12516225635, vstest.console.exe, TestRequestSender: Aborting test discovery because Error setting value to 'StoreKeyValuePairs' on 'Microsoft.VisualStudio.TestPlatform.ObjectModel.TestCase'.

[smadala]

This was fixed with #1640