check-meta: filter nulls out of {maintainers,teams}Position

Note that Nix checks for this when it's querying meta:

https://github.com/NixOS/nix/blob/2.18.8/src/nix-env/nix-env.cc#L963

pkgs/stdenv/generic/check-meta.nix

@@ -26,6 +26,7 @@ let
     isAttrs
     isString
     mapAttrs
+    filterAttrs
     ;
 
   inherit (lib.lists)
@@ -593,11 +594,12 @@ let
         )
       ] ++ optional (hasOutput "man") "man";
     }
-    // {
-      # CI scripts look at these to determine pings.
+    // (filterAttrs (_: v: v != null) {
+      # CI scripts look at these to determine pings. Note that we should filter nulls out of this,
+      # or nix-env complains: https://github.com/NixOS/nix/blob/2.18.8/src/nix-env/nix-env.cc#L963
       maintainersPosition = builtins.unsafeGetAttrPos "maintainers" (attrs.meta or { });
       teamsPosition = builtins.unsafeGetAttrPos "teams" (attrs.meta or { });
-    }
+    })
     // attrs.meta or { }
     # Fill `meta.position` to identify the source location of the package.
     // optionalAttrs (pos != null) {