weblate: skip null packages in relaxDeps logic

This fixes eval on staging, because there Python 3.14 is the default and
backports-zstd is null, because it is included by default.

(cherry picked from commit 0fd69dc26c)

pkgs/by-name/we/weblate/package.nix

@@ -111,7 +111,7 @@ python3Packages.buildPythonApplication (finalAttrs: {
       ];
     in
     lib.concatMap (
-      p: if lib.elem p.pname coreDeps then [ ] else [ p.pname ]
+      p: if p == null || lib.elem p.pname coreDeps then [ ] else [ p.pname ]
     ) finalAttrs.passthru.dependencies;
 
   dependencies =