`eth_getBlockReceipts` should fail on null rounds

[LesnyRumcajs]

Summary

Right now, Forest follows Lotus so returns previous tipset's receipts for eth_getBlockReceipts in case the param points to a null round. This is correct from API parity POV, but not really consistent across other methods fixed in #7276.

Completion Criteria

• Forest returns null error on null rounds for eth_getBlockReceipts and eth_getBlockReceiptsLimited unless a dedicated environmental variable is set
• Fallback to legacy behavior if set

Additional Links & Resources

Related filecoin-project/lotus#12633