Fix an error while running db_crashtest for non-user-ts tests (#8091)

Summary:
Fix the following error while running `make crash_test`
```
Traceback (most recent call last):
  File "tools/db_crashtest.py", line 705, in <module>
    main()
  File "tools/db_crashtest.py", line 696, in main
    blackbox_crash_main(args, unknown_args)
  File "tools/db_crashtest.py", line 479, in blackbox_crash_main
    + list({'db': dbname}.items())), unknown_args)
  File "tools/db_crashtest.py", line 414, in gen_cmd
    finalzied_params = finalize_and_sanitize(params)
  File "tools/db_crashtest.py", line 331, in finalize_and_sanitize
    dest_params.get("user_timestamp_size") > 0):
TypeError: '>' not supported between instances of 'NoneType' and 'int'
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8091

Test Plan: make crash_test

Reviewed By: ltamasi

Differential Revision: D27268276

Pulled By: riversand963

fbshipit-source-id: ed2873b9587ecc51e24abc35ef2bd3d91fb1ed1b

tools/db_crashtest.py

@@ -142,6 +142,7 @@ default_params = {
     "paranoid_file_checks": lambda: random.choice([0, 1, 1, 1]),
     "max_write_buffer_size_to_maintain": lambda: random.choice(
         [0, 1024 * 1024, 2 * 1024 * 1024, 4 * 1024 * 1024, 8 * 1024 * 1024]),
+    "user_timestamp_size": 0,
 }
 
 _TEST_DIR_ENV_VAR = 'TEST_TMPDIR'