KNetProducer stores a wrong timestamp if not explicitly set

[masesdevelopers]

Describe the bug
The timestamp stored in a topic is not set to current time, but reports the UNIX epoch time.

To Reproduce
Steps to reproduce the behavior:

1. Allocates a KNetProducer
2. Produce a KNetProducerRecord<TKey, TValue>(topic, key, value)
3. Get data from the topic and see the timestamp

Expected behavior
The timestamp shall be set to the current time if not explicitly set in KNetProducerRecord

Screenshots
No screenshot

Desktop (please complete the following information):
Latest KNet release

Additional context
N/A

[masesdevelopers]

To inform Apache Kafka to associate a timestamp to the record, the corresponding data in ProducerRecord shall be null since the timestamp is a Java.Lang.Long which can be a nullable type.
When the conversion is done here

KNet/src/net/KNet/Specific/Producer/KNetProducer.cs

Line 346 in ab6be44

return new ProducerRecord<byte[], byte[]>(record.Topic, record.Partition, record.Timestamp,

the original type is not a nullable type since this is the declaration

KNet/src/net/KNet/Specific/Producer/KNetProducer.cs

Line 135 in ab6be44

public long Timestamp { get; private set; }

To avoid the problem both Timestamp and Partition shall be converted into .NET nullable types. A nullable Partition inform Apache Kafka to choose the partition, if the topic have multiple one, using the partition strategy in use, otherwise always the default one (0 for an int) will be used.