tests: use __empty and __raw

Signed-off-by: saygo-png <saygo.mail@proton.me>

tests/test-sources/plugins/by-name/coverage/default.nix

@@ -87,7 +87,7 @@
             coverage_file = "coverage/coverage.json";
           };
         };
-        lcov_file = null;
+        lcov_file.__raw = "nil";
       };
     };
   };