Warp sync doesn't work if the target block is the genesis

[tmpolaczyk]

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Description of bug

It is not possible to bootstrap a chain when all the collators are using --sync warp. This seems to be because warp sync doesn't work when the target block is the genesis. I found this bug when trying to sync to a parachain, but it also affects the relaychain.

I see these logs in a loop: "Waiting for 3 peers to be connected" but there are 5 peers already.

2023-10-18 15:31:00 🙌 Starting consensus session on top of parent 0xe6f9d4c36cc368ae78763b852fa074810ef19a603b18009432cf711813a5167d    
2023-10-18 15:31:02 ParentBlockRandomness did not provide entropy    
2023-10-18 15:31:02 ParentBlockRandomness did not provide entropy    
2023-10-18 15:31:02 🎁 Prepared block for proposing at 1 (0 ms) [hash: 0x0acb8da043e4079ba1cadb2813f6ed0c2676375576a3e01e82cd7384c7a56fd9; parent_hash: 0xe6f9…167d; extrinsics (2): [0xe708…6936, 0x770f…0e89]    
2023-10-18 15:31:02 🔖 Pre-sealed block for proposal at 1. Hash now 0x8d263a9ebea83e8420c9060ed91cf9083d761c66f94e8eb8fa1111a5293a48c1, previously 0x0acb8da043e4079ba1cadb2813f6ed0c2676375576a3e01e82cd7384c7a56fd9.    
2023-10-18 15:31:02 👶 New epoch 0 launching at block 0x8d26…48c1 (block slot 282939310 >= start slot 282939310).    
2023-10-18 15:31:02 👶 Next epoch starts at slot 282939320    
2023-10-18 15:31:03 ⏩ Warping, Waiting for 3 peers to be connected, 0.00 Mib (5 peers), best: #0 (0xe6f9…167d), finalized #0 (0xe6f9…167d), ⬇ 0.4kiB/s ⬆ 1.2kiB/s    
2023-10-18 15:31:08 ⏩ Warping, Waiting for 3 peers to be connected, 0.00 Mib (5 peers), best: #0 (0xe6f9…167d), finalized #0 (0xe6f9…167d), ⬇ 1.4kiB/s ⬆ 0.9kiB/s    
2023-10-18 15:31:12 🙌 Starting consensus session on top of parent 0xe6f9d4c36cc368ae78763b852fa074810ef19a603b18009432cf711813a5167d    

Steps to reproduce

./polkadot build-spec --chain rococo-local --disable-default-bootnode --raw > rococo-local-cfde.json
./polkadot --chain rococo-local-cfde.json --tmp --sync warp --no-beefy --alice --port 30331
./polkadot --chain rococo-local-cfde.json --tmp --sync warp --no-beefy --bob --port 30332

You can add more nodes (I had 6), but it doesn't seem to fix it. But if alice and bob are set to --sync full, the other nodes will eventually sync using warp sync, when the target block is greater than 0.