profiles: Hooks always return a monadic value.

Suggested by 宋文武 <iyzsong@gmail.com>.

* guix/profiles.scm (ghc-package-cache-file): Return a monadic #f, not just
  #f, when MANIFEST contains no GHC package.
  (profile-derivation): Do not filter based on the return value of HOOK;
  instead filter based on the value of items in EXTRAS.

guix/profiles.scm

@@ -516,12 +516,14 @@ entries of MANIFEST, or #f if MANIFEST does not have any GHC packages."
           (for-each delete-file (find-files db-dir "\\.conf$"))
           success)))
 
+  (with-monad %store-monad
     ;; Don't depend on GHC when there's nothing to do.
-  (and (any (cut string-prefix? "ghc" <>)
+    (if (any (cut string-prefix? "ghc" <>)
              (map manifest-entry-name (manifest-entries manifest)))
         (gexp->derivation "ghc-package-cache" build
                           #:modules '((guix build utils))
-                         #:local-build? #t)))
+                          #:local-build? #t)
+        (return #f))))
 
 (define (ca-certificate-bundle manifest)
   "Return a derivation that builds a single-file bundle containing the CA
@@ -602,11 +604,14 @@ the monadic procedures listed in HOOKS--such as an Info 'dir' file, etc."
   (mlet %store-monad ((extras (if (null? (manifest-entries manifest))
                                   (return '())
                                   (sequence %store-monad
-                                            (filter-map (lambda (hook)
+                                            (map (lambda (hook)
                                                    (hook manifest))
                                                  hooks)))))
     (define inputs
-      (append (map gexp-input extras)
+      (append (filter-map (lambda (gexp)
+                            (and (gexp? gexp)
+                                 (gexp-input gexp)))
+                          extras)
               (manifest-inputs manifest)))
 
     (define builder