When using checkpoint sync, the initial block is missing in the DB.
Update the LC data collector initialization to account for that,
avoiding a spurious error message when it is incorrectly accessed:
```
ERR 2024-02-07 11:21:55.416+01:00 Block failed to load unexpectedly topics="chaindag_lc" bid=d30517a7:8257504 tail=8257504
```
Also fixes a regression from #5691 that resulted in similar messages
while importing the first few blocks after checkpoint sync.
Thanks to @arnetheduck for reporting this.
94ba0a9bd1