fix a panic on startup when we fail to load the tipset

[Stebalien]

There may be a deeper issue here, but we need to handle this more gracefully somehow. See #4358.

[Stebalien]

An alternative is to turn the following into a hard error:

lotus/node/modules/chain.go

Lines 95 to 97 in b61281b

	if err := chain.Load(); err != nil {
	log.Warnf("loading chain state from disk: %s", err)
	}

But even then, we should still handle the case where no tipset has been loaded.