formula_cellar_checks: skip unless Python libs

This avoids an unusual error message when Python is needed for building
(e.g., code generation) but is not used at runtime.

```
==> brew audit recode --online
Error: 1 problem in 1 formula detected
recode:
  * Packages have been installed for:

    but this formula depends on:
      Python 3.7
```

Library/Homebrew/formula_cellar_checks.rb

@@ -180,6 +180,8 @@ module FormulaCellarChecks
       match.captures.first
     end.compact
 
+    return if pythons.blank?
+
     python_deps = deps.map(&:name)
                       .grep(/^python(@.*)?$/)
                       .map { |d| Formula[d].version.to_s[/^\d+\.\d+/] }