ast: Return an error when parsing an empty module

[patrick-east]

The documentation is pretty clear that a module must at least contain
a package, so it is safe to say that an empty file isn't valid.

Previously the helper would just return a nil module and nil error, it
will now return an error.

Fixes: #2054
Signed-off-by: Patrick East east.patrick@gmail.com

[tsandall]

Maybe you've already done this but it would be good to review the callers of ParseModule to remove any unnecessary checks for the nil, nil case now. I'm guessing most weren't doing it properly since this is not an obvious return value.

Other than that, since this change is backwards incompatible, let's put a small release note in place.

[patrick-east]

it would be good to review the callers of ParseModule to remove any unnecessary checks for the nil, nil case now

Done, and removed a handful of spots.

I'm guessing most weren't doing it properly since this is not an obvious return value.

Yea.. it was a very small minority that would check for it.

[tsandall]

Let's merge this after we cut the v0.17.2 release to minimize the impact.

[tsandall]

@patrick-east let's merge this now that v0.17.2 is out. Just need to resolve the changelog conflict..