Fix toplevel_stanzas.

[reitermarkus]

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Not sure if this fixes anything for the other PRs, but it should at least make tests behave more consistently when using only a single stanza.

[BrewTestBot]

Review period will end on 2023-03-22 at 13:45:47 UTC.

[issyl0]

I'm not sure I understand how this fixes it, but as you say it might not.

[reitermarkus]

It will probably fix the tests in your PR since those only contain one stanza. So in

cask do
  on_monterey do
    url 
  end
end

url would previously be considered a top-level stanza, because on_monterey is the only stanza in cask.

[issyl0]

Ohhhh right, I get it, because of the (great!) explanation you gave of the AST parsing in #15013 (comment). Thanks!