Revert "Return nulls when TimeSent is missing"

[weralabaj]

Reverts #491

[mikeminutillo]

Doing a deeper investigation I came across this.

Whenever we construct (at least in v4.6 on NServiceBus) a TransportMessage the TimeSent header is filled in to UtcNow. https://github.com/Particular/NServiceBus/blob/support-4.6/src/NServiceBus.Core/TransportMessage.cs#L32

That means when we ingest an error that does not have that error set here, it is set to the time of ingestion.

It should never be null.

[weralabaj]

@WolfByte Exactly it was done in Jan 2014. But this is only applied for outgoing messages (see usages of this constructor), the problem is for messages coming from external systems to NSB.