gpg: Allow setting notations with the empty string as value.

* g10/misc.c (pct_expando): Catch special case of the empty string.
Also map a NULL to the empty string.
* g10/photoid.c (show_photos): Make an empty string used as command
fail.
--

This patch also fixes a segv when calling gpg wrongly like

  gpg -N \-foo@example.org

GnuPG-bug-id: 5117
Signed-off-by: Werner Koch <wk@gnupg.org>

g10/misc.c

@@ -922,6 +922,11 @@ pct_expando (ctrl_t ctrl, const char *string,struct expando_args *args)
   u32 pk_keyid[2]={0,0},sk_keyid[2]={0,0};
   char *ret=NULL;
 
+  /* The parser below would return NULL for an empty string, thus we
+   * catch it here.  Also catch NULL here. */
+  if (!string || !*string)
+    return xstrdup ("");
+
   if(args->pk)
     keyid_from_pk(args->pk,pk_keyid);
 

g10/photoid.c

@@ -670,6 +670,11 @@ show_photos (ctrl_t ctrl, const struct user_attribute *attrs, int count,
 	command = pct_expando (ctrl, opt.photo_viewer,&args);
 	if(!command)
 	  goto fail;
+        if (!*command)
+          {
+            xfree (command);
+            goto fail;
+          }
 
 	name = xmalloc (1 + 16 + strlen(EXTSEP_S)
                         + strlen (image_type_to_string (args.imagetype, 0)));