Adjust CheckBlockIndex to account for
- assumed-valid block indexes lacking transaction data, and
- setBlockIndexCandidates for the background chainstate not containing certain entries
which rely on assumed-valid ancestors.
This is a backport of core#21526 [8/12]
https://github.com/bitcoin/bitcoin/pull/21526/commits/8f5710fd0ac5173b577e5d00708485170b321bcc
Depends on D12280