Fixed ArrrayConverter serialization of nullable types

[nils2525]

Fixed an error I encountered when trying to serialise the Nullable<bool> property BitfinexTradeDetails.Maker.

  System.Text.Json.JsonReaderException: 'T' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0.
   at System.Text.Json.ThrowHelper.ThrowJsonReaderException(Utf8JsonReader& json, ExceptionResource resource, Byte nextByte, ReadOnlySpan`1 bytes)
   at System.Text.Json.Utf8JsonReader.ConsumeValue(Byte marker)
   at System.Text.Json.Utf8JsonReader.ReadFirstToken(Byte first)
   at System.Text.Json.Utf8JsonReader.ReadSingleSegment()
   at System.Text.Json.Utf8JsonReader.Read()
   at System.Text.Json.Utf8JsonWriter.WriteRawValueCore(ReadOnlySpan`1 utf8Json, Boolean skipInputValidation)
   at System.Text.Json.Utf8JsonWriter.TranscodeAndWriteRawValue(ReadOnlySpan`1 json, Boolean skipInputValidation)
   at System.Text.Json.Utf8JsonWriter.WriteRawValue(String json, Boolean skipInputValidation)
   at CryptoExchange.Net.Converters.SystemTextJson.ArrayConverter.ArrayConverterInner`1.Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options) in C:\Dev\CryptoExchangeNet\CryptoExchange.Net\CryptoExchange.Net\Converters\SystemTextJson\ArrayConverter.cs:line 97
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.WriteCore(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.Serialize(Utf8JsonWriter writer, T& rootValue, Object rootValueBoxed)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.SerializeAsObject(Utf8JsonWriter writer, Object rootValue)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.Serialize(Utf8JsonWriter writer, T& rootValue, Object rootValueBoxed)
   at System.Text.Json.JsonSerializer.WriteString[TValue](TValue& value, JsonTypeInfo`1 jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)

[JKorf]

Hi, there was already a cached property for the underlying type, so I used that to instead of getting the underlying type each time. Thanks for bringing this to my attention. (and your contributions in the other libraries)