dev-cmd/unbottled: Fail nicely if analytics data can't be retrieved

- When testing `brew unbottled <formula>` this crashed because I, as a
  maintainer, have `HOMEBREW_NO_ANALYTICS` set on all my machines to
  avoid polluting the analytics with test installs.

```
❯ brew unbottled
==> Getting formulae...
==> Getting analytics data...
Error: undefined method `[]' for nil:NilClass
/usr/local/Homebrew/Library/Homebrew/dev-cmd/unbottled.rb:89:in `formulae_all_sort_installs_from_args'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/unbottled.rb:49:in `unbottled'
/usr/local/Homebrew/Library/Homebrew/brew.rb:119:in `<main>'
```

Library/Homebrew/dev-cmd/unbottled.rb

@@ -86,6 +86,13 @@ module Homebrew
 
       ohai "Getting analytics data..."
       analytics = Utils::Analytics.formulae_brew_sh_json("analytics/install/90d.json")
+
+      if analytics.blank?
+        raise UsageError,
+              "default sort by analytics data requires " \
+              "`HOMEBREW_NO_GITHUB_API` and `HOMEBREW_NO_ANALYTICS` to be unset"
+      end
+
       formulae = analytics["items"].map do |i|
         f = i["formula"].split.first
         next if f.include?("/")