Fix segfault on TPCH benchmark shutdown

[sergei-lewis]

shutdown() calls cache_->shutdown() unconditionally; however, if ssd_cache_gb was not provided, the cache is not initialised and the null pointer dereference causes a segfault.

Check cache_ is initialised before interacting with it, as is done everywhere else in the module.

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[majetideepak]

@sergei-lewis can you add a CI test so that this will not break again?
There is a issue for this here #6834

[sergei-lewis]

@sergei-lewis can you add a CI test so that this will not break again? There is a issue for this here #6834

Hm, running the benchmark requires a directory structure containing a parquet tpch dataset; looking through the codebase, dbgen/tpchgen interact with the velox api but there's no existing tool or wrapper for triggering these in CI to generate a suitable dataset, so we'd need to write one. I'll see if I can get some time to work out how to do that at some point.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. If you'd still like this PR merged, please comment on the PR, make sure you've addressed reviewer comments, and rebase on the latest main. Thank you for your contributions!