Revert "lib.meta.availableOn: Return false if pkg parameter is null" (#410179)

lib/meta.nix

···
   */
   availableOn =
     platform: pkg:
-    pkg != null
-    && ((!pkg ? meta.platforms) || any (platformMatch platform) pkg.meta.platforms)
+    ((!pkg ? meta.platforms) || any (platformMatch platform) pkg.meta.platforms)
     && all (elem: !platformMatch platform elem) (pkg.meta.badPlatforms or [ ]);
 
   /**