nixos/screen: fix assertion to actually execute

See NixOS#312194 (comment) for explanation why the assertion currently fails to run.

nixos/modules/programs/screen.nix

@@ -12,7 +12,8 @@ in
       package = lib.mkPackageOptionMD pkgs "screen" { };
 
       screenrc = lib.mkOption {
-        type = with lib.types; nullOr lines;
+        type = lib.types.lines;
+        default = "";
         example = ''
           defscrollback 10000
           startup_message off
@@ -22,20 +23,22 @@ in
     };
   };
 
-  config = {
-    # TODO: Added in 24.05, remove before 24.11
-    assertions = [
-      {
-        assertion = cfg.screenrc != null -> cfg.enable;
-        message = "`programs.screen.screenrc` has been configured, but `programs.screen.enable` is not true";
-      }
-    ];
-  } // lib.mkIf cfg.enable {
-    environment.etc.screenrc = {
-      enable = cfg.screenrc != null;
-      text = cfg.screenrc;
-    };
-    environment.systemPackages = [ cfg.package ];
-    security.pam.services.screen = {};
-  };
+  config = lib.mkMerge [
+    {
+      # TODO: Added in 24.05, remove before 24.11
+      assertions = [
+        {
+          assertion = cfg.screenrc != "" -> cfg.enable;
+          message = "`programs.screen.screenrc` has been configured, but `programs.screen.enable` is not true";
+        }
+      ];
+    }
+    (lib.mkIf cfg.enable {
+      environment.etc.screenrc = {
+        text = cfg.screenrc;
+      };
+      environment.systemPackages = [ cfg.package ];
+      security.pam.services.screen = {};
+    })
+  ];
 }