raco pkg config: treat empty-string catalog as default paths

An empty string provided to `raco pkg config --set catalogs` would
trigger an error previously. Instead, turn it into a `#f` in the
configuration file, which is replaced by the default search sequence.

(cherry picked from commit ec0350e6d9)

pkgs/racket-doc/pkg/scribblings/pkg.scrbl

@@ -897,11 +897,15 @@ for @nonterm{key}.
  @item{@DFlag{scope-dir} @nonterm{dir} --- Same as for @command-ref{install}.}
  ]
 
- The valid @nonterm{key}s are:
+ The valid @nonterm{key}s and corresponding @nonterm{val}s are:
  @itemlist[
   @item{@exec{name} --- A string for the installation's name, which is used by @exec{user}
         @tech{package scope} and defaults to the Racket version.}
-  @item{@exec{catalogs} --- A list of URLs for @tech{package catalogs}.}
+  @item{@exec{catalogs} --- A list of URLs for @tech{package catalogs}. An empty-string
+        @nonterm{val} is replaced by the sequence of catalogs for the default configuration.
+        A @nonterm{val} that does not start with alphabetic characters followed by @litchar{://}
+        is treated as a path relative to the configuration directory (as
+        reported by @racket[find-config-dir]).}
   @item{@exec{default-scope} --- Either @exec{installation} or @exec{user}.
         The value of this key at @exec{user} scope (possibly defaulting from
         @exec{installation} scope) is

racket/collects/pkg/private/config.rkt

@@ -144,7 +144,11 @@
                    key
                    (format-list (cons val more-vals)))]
        [(list* (and key "catalogs") val)
-        (update-pkg-cfg! 'catalogs val)]
+        (update-pkg-cfg! 'catalogs
+                         (for/list ([s (in-list val)])
+                           (cond
+                            [(equal? s "") #f]
+                            [else s])))]
        [(list (and key "default-scope") val)
         (unless (member val '("installation" "user"))
           (pkg-error (~a "invalid value for config key\n"