lib.makeSearchPath: allow null in search path

This makes things match ‘buildInputs’ where inputs are allowed to be
null.

lib/strings.nix

···
        => "//bin"
   */
   makeSearchPath = subDir: packages:
-    concatStringsSep ":" (map (path: path + "/" + subDir) packages);
+    concatStringsSep ":" (map (path: path + "/" + subDir) (builtins.filter (x: x != null) packages));
 
   /* Construct a Unix-style search path, using given package output.
      If no output is found, fallback to `.out` and then to the default.