block header cache not initialized before used

[lu-pinto]

PR description

After this commit the block headers cache is being used before it's initialized which creates an NPE while setting up the metrics:

Caused by: java.lang.NullPointerException: Cannot invoke \"java.util.Optional.map(java.util.function.Function)\" because \"this.blockHeadersCache\" is null
    at org.hyperledger.besu.ethereum.chain.DefaultBlockchain.getBlockHeader(DefaultBlockchain.java:328)
    at java.base/java.util.Optional.flatMap(Optional.java:289)
    at org.hyperledger.besu.ethereum.chain.DefaultBlockchain.getFinalizedBlockNumber(DefaultBlockchain.java:775)
    at org.hyperledger.besu.plugin.services.MetricsSystem.lambda$createLongGauge$1(MetricsSystem.java:160)
    at org.hyperledger.besu.metrics.prometheus.CurrentValueCollector.collect(CurrentValueCollector.java:55)
    ...

Fixed Issue(s)

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests

[garyschulte]

🚢