Clean up RPC responses for pruned blocks

[Matilda-Clerke]

As a caller of Besu's RPC endpoints, I want Besu to return sensible results when queried for blocks which have been pruned.

1. Investigate the appropriate response (empty vs error code 4444?)
2. Utilise Geth's workload test tool to identify all RPC endpoiints needing updating
3. Update code to match expectations