Merge pull request #5594 from MikeMcQuaid/style-handle-files

style: better handle file arguments.

Author: MikeMcQuaid

Library/Homebrew/style.rb

@@ -61,7 +61,7 @@ def check_style_impl(files, output_type, options = {})
       end
 
       if files && !has_non_formula
-        config = if (files.first/"spec").exist?
+        config = if files.first && File.exist?("#{files.first}/spec")
           HOMEBREW_LIBRARY/".rubocop_rspec.yml"
         else
           HOMEBREW_LIBRARY/".rubocop_audit.yml"