[Chronik] Fix reading coinbase txs when plugins are loaded

Summary:
Currently, if any plugins are loaded, APIs reading txs fail for coinbase txs.

This is because MempoolPlugins::fetch_plugin_outputs does a consistency check for missing inputs, and coinbase txs indeed don't have existing inputs, so we have to special case them.

Test Plan: ./test/functional/chronik_plugins.py

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16568