blockchain/v2: don't broadcast base if height is 0
## Description
Fixes a bug where the reactor would broadcast a base with height=0.
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For contributor use:
- [ ] Wrote tests
- [ ] Updated CHANGELOG_PENDING.md
- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] Updated relevant documentation (`docs/`) and code comments
- [x] Re-reviewed `Files changed` in the Github PR explorer