Merge pull request #2652 from ewang12/fix-brew-cask-no-args

Fix implicit conversion of nil into string error

Library/Homebrew/brew.rb

@@ -21,6 +21,7 @@ if ARGV == %w[--version] || ARGV == %w[-v]
 end
 
 def require?(path)
+  return false if path.nil?
   require path
 rescue LoadError => e
   # we should raise on syntax errors but not if the file doesn't exist.