Create EthAccount at 0x00..00 + fix default sender address

[raulk]

This PR made us default to the system actor address in masked ID form as a sender when no from address is provided to eth_call. That address is 0xff00..00. However, clients expect this address to be 0x00..00 (the null Ethereum address). We need to do two things here:

1. Create an EthAccount actor with an f410 address equivalent to 0x00..00. The actor ID does not matter. See how Lotus does that here.
2. Fix eth_call to use that address as the default sender.

[raulk]

@cryptoAtwill are you a good person to take this on?

[aakoshh]

However, clients expect this address to be 0x00..00

@raulk where exactly do they expect to see this? Where do they observe 0xff00..00?

[raulk]

Say, for example, that a contract returns the sender address as a return value. Clients will expect that address to be 0x00..00 when the "from" field was elided. It's a long-standing convention in Eth APIs that can break random things down the line if not honoured.