Fix an incorrect assertion when the block locator is at the tip

[teor2345]

Motivation

Zebra panics when it's at the tip, and it's asked for a block locator for that tip.
(This might depend on whether there's a stop height, and whether integer checks are optimised out.)

This might have been triggered by receiving block hash gossips from the new Zebra code in PR #2729.

Solution

• Skip the assertion when the response is empty
• Add tests for BlockLocator, FindBlockHashes, and FindBlockHeaders
• Add a specific test for this bug

Review

Anyone can review this panic fix, but I think @oxarbitrage might have looked at this code recently?

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[teor2345]

The macOS failure looks like a spurious timing issue:

Message: Opening database "/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/zebrad_tests.qiCrlWIEdGK7/state/state/v10/mainnet" failed: Error { message: "IO error: While lock file: /var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/zebrad_tests.qiCrlWIEdGK7/state/state/v10/mainnet/LOCK: Resource temporarily unavailable" }. Hint: Check if another zebrad process is running. Try changing the state cache_dir in the Zebra config.

https://github.com/ZcashFoundation/zebra/pull/2789/checks?check_run_id=3692472245#step:10:1326

[oxarbitrage]

Looks good. Just a minor suggestion in a doc comment.