Handle nil task resources consistently

[ryanuber]

We have some spots (maybe even just docker) where we panic if the provided task resources is a typed nil. We should either handle this uniformly before invoking a specific driver, or remove the panic.

Example: https://github.com/hashicorp/nomad/blob/master/client/driver/docker.go#L65-L67

[cbednarski]

I removed this. Closing for now!

[github-actions]

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