compat/extend/string: fix odisabled return.

`odisabled` will still return for a formula in `.brew`. This means
`EOS.undent` returns `nil` and the formula cannot be parsed. Instead
return the actual string in this case to avoid e.g. patches blowing up
with `nil` strings.

Fixes #4049.

Library/Homebrew/compat/extend/string.rb

@@ -1,6 +1,7 @@
 class String
   def undent
     odisabled "<<-EOS.undent", "<<~EOS"
+    self
   end
   alias unindent undent