melpa-packages: remove condition about null in helper functions

This is a follow up of [1].  With that PR applied, packages will not
be null.

[1]: https://github.com/NixOS/nixpkgs/pull/83378

pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix

@@ -31,36 +31,34 @@ in
 { lib, pkgs }: variant: self:
 let
   dontConfigure = pkg:
-    if pkg != null then pkg.override (args: {
+    pkg.override (args: {
       melpaBuild = drv: args.melpaBuild (drv // {
         dontConfigure = true;
       });
-    }) else null;
+    });
 
   markBroken = pkg:
-    if pkg != null then pkg.override (args: {
+    pkg.override (args: {
       melpaBuild = drv: args.melpaBuild (drv // {
         meta = (drv.meta or { }) // { broken = true; };
       });
-    }) else null;
+    });
 
   externalSrc = pkg: epkg:
-    if pkg != null then pkg.override (args: {
+    pkg.override (args: {
       melpaBuild = drv: args.melpaBuild (drv // {
         inherit (epkg) src version;
 
         propagatedUserEnvPkgs = [ epkg ];
       });
-    }) else null;
+    });
 
   buildWithGit = pkg: pkg.overrideAttrs (attrs: {
     nativeBuildInputs =
       (attrs.nativeBuildInputs or [ ]) ++ [ pkgs.git ];
   });
 
-  fix-rtags = pkg:
-    if pkg != null then dontConfigure (externalSrc pkg pkgs.rtags)
-    else null;
+  fix-rtags = pkg: dontConfigure (externalSrc pkg pkgs.rtags);
 
   generateMelpa = lib.makeOverridable ({ archiveJson ? defaultArchive
                                        }: