Timestamp supplied to FVM around null rounds is wrong / needs clarification

[arajasek]

As currently implemented in the reference implementation, the FVM is created with a "timestamp" always corresponding to the non-null round being executed. However, this gets tricky around null rounds, which still have execution (cron) that needs to occur. We need to:

• clarify what the correct behaviour is (see here)
• Fixup clients if necessary (likely necessary).

Note that this doesn't actually have any consequences right now, because none of the Cron operations query the timestamp, but we should align and fixup.

[raulk]

Fix merged in filecoin-project/lotus#10189.