Fix #314 - Timestamp property throws ArgumentNullException

Created by: Kiritsu

Summary

Sometimes, TimestampRaw is null (message not in cache?), so the method DatetimeOffset.Parse would throw an ArgumentNullException. But we have the property CreationTimestamp in SnowflakeObject that is set with the Id of the snowflake, so, in case TimestampRaw is null, we just return the value of CreationTimestamp, if TimestampRaw is not null, then we try to parse and return it.

Details

Simply check if the property TimestampRaw is null, then TryParse instead of Parse (to avoid exceptions), and in cases we can't parse and the property is null, we can secure Timestamp property from not being null by taking CreationTimestamp value.