fix: eth: avoid panicing on start if we don't have a head

[Stebalien]

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Related Issues

Proposed Changes

Check if we actually have a head before trying to pre-fill the tipset cache in the Eth module

This case shouldn't happen unless the node gets corrupted (I think?). Or maybe when starting at genesis? But there's no reason to panic regardless.

Additional Info

This really isn't the right place to do this prefill, but I don't know where it should be moved. We could move it, e.g., into ChainStore.Start, but then we'd run this on all nodes. Is that OK?

Checklist

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[Stebalien]

This isn't sufficient as the node will just fail when it tries to call Start.

[Stebalien]

Testing an alternative in #11426.