fix(providers): receipts of unmined blocks should be null

[kallydev]

Motivation

The provider cannot distinguish between non-existent blocks and blocks without transactions when using the eth_getBlockReceipts method.

RPC responses

• The block exists, but there are no transactions.

{
  "jsonrpc": "2.0",
  "id": 0,
  "result": []
}

• The block does not exist.

{
  "jsonrpc": "2.0",
  "id": 0,
  "result": null
}

Solution

Use Option to distinguish between [] and null.

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[mattsse]

I think this is reasonable and the right fix

pending @Evalir

[Evalir]

yep this looks good, agreed this is a good fix.