setup-scribblings: guard against missing 'scribblings

Since `get-info/full` may return a function that is undefined
 for the symbol 'scribblings, pass a failure thunk.

The function can be undefined for 'scribblings if:

1. Install a package with a `scribblings` in its info.rkt
2. Remove `scribblings` from the info file
3. Run `raco setup`

pkgs/racket-index/setup/scribble.rkt

@@ -180,7 +180,7 @@
                            infos)])
            (and (not (memq #f infos)) infos))))
   (define ((get-docs main-dirs) i rec)
-    (let* ([pre-s (and i (i 'scribblings))]
+    (let* ([pre-s (and i (i 'scribblings (λ () #f)))]
            [s (validate-scribblings-infos pre-s)]
            [dir (directory-record-path rec)])
       (if s