Potential bug with Casper node 1.5: Failure to deserialize the System proposed switch block

[mrkara]

Try to deserialize the "System proposed switch block" -> the .Net SDK fails because the Proposer is normally a PublicKey but now it is "00" which is invalid.

The system-propsoed switch block: https://cspr.live/block/1562c81ea1623b6c882370b4a659b797e06e30ddbc0053b6b0d3a1aef71c6d33

First discovered on the .NET SDK: make-software/casper-net-sdk#37