Respond to getblockchaininfo with genesis block when empty state

[elijahhampton]

Motivation

A lightwalletd test fails for getblockchaininfo when the state is empty. This PR refactors the rpc request to return the genesis block.

Specifications & References

Solution

Adds a test for returning the genesis state in prop.rs. Refactors getblockchaininfo rpc to return the genesis block if TipPoolValues request fails.

Tests

Follow-up Work

PR Author's Checklist

• [X ] The PR name will make sense to users.
• [X ] The PR provides a CHANGELOG summary.
• [X ] The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.