context Refresh and Apply sometimes return nil

[jbardin]

The documentation for Refresh indicates that it will always return a
valid state, but that wasn't true in the case of a graph builder error.
While this same concept wasn't documented for Apply, it was still
assumed in the terraform apply code.

Since the helper testing framework relies on the absence of a state to
determine if it can call Destroy, the Context can't can't start
returning a state in all cases. Document this, and use the State method
to fetch the correct state value after Apply.

Add a nil check to the WriteState function, so that writing a nil state
is a noop.

Make sure to init before sorting the state, to make sure we're not
attempting to sort nil values. This isn't technically needed with the
current code, but it's just safer in general.

Fixes #13565

[apparentlymart]

It looks like a couple tests are failing with a nil pointer dereference error here, though not enough context in the Travis log for me to guess whether it was actually caused by this change. Hard to imagine how this change would cause it unless there are cases where these methods are called on a nil Context...

[jbardin]

@apparentlymart: interesting. I'll take a look locally - I admit I delegated testing this to travis ;)

[apparentlymart]

Looks like in the long run there's some cleanup worth doing here to limit how much these nil states get passed around, but this seems like a fine short-term change to keep us rolling!

[ghost]

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