When there is no explicit configuration for the current fork, use the last
specified fork value. If no last value is specified, set both to zero.
Rationale
Although maintaining the target and max blob only in the consensus client is
desirable, we acknowledge the reality that execution clients need these values
for various activities. For example, the eth_feeHistory RPC method returns a
field blobGasUsedRatio that does require the max, even though the core
protocol doesn’t specifically need such value. Passing this value over the
engine API every block seem overkill so we believe a configuration value is a
good middle ground.