JSON error when mining unverified transactions inside of block

[dlowrey]

To reproduce:

• delete data directory
• run main.py
• type send test 100
• type start

The mining crashes when trying to gather the unverified transactions for the new block because the file verified_transactions.json was created with {}, the same for utxo.json. This causes json.load() to crash with json.decoder.JSONDecodeError: Extra data: line 1 column 3 (char 2) (because of the extra {})