In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw3d 
scroll bars)
  of 2015-09-05
Repository revision: 2330ca33a97867f2ea1123bcf7bfe5cfcc030b36


rgrep's `grep-find-ignored-directories' conditional-ignore behaviour
(for list elements which are cons cells) produces 'find' constraints
like: "find . -type d \( -path  -o -path \*/SCCS [...]" (i.e. a -path
argument with no value) when the associated function returns nil.

The nil elements need to be deleted from the list before the command
line arguments for the ignored directories are generated.