perf: Skip Cache allocation for the common case

Signed-off-by: Dmitry Dygalo <dmitry@dygalo.dev>

Author: Stranger6667

src/harfile/__init__.py

@@ -147,15 +147,17 @@ def add_entry(
         dumps = _encode
         if isinstance(startedDateTime, datetime):
             startedDateTime = startedDateTime.isoformat()
-        cache = cache or Cache()
+        # The common case is no cache info; avoid allocating a Cache and
+        # encoding an empty dict (``dumps(Cache().asdict())`` is just ``"{}"``).
+        cache_json = "{}" if cache is None else dumps(cache.asdict())
         chunk = (
             f"{separator}            {{"
             f'\n                "startedDateTime": "{startedDateTime}",'
             f'\n                "time": {time},'
             f'\n                "request": {dumps(request.asdict())},'
             f'\n                "response": {dumps(response.asdict())},'
             f'\n                "timings": {dumps(timings.asdict())},'
-            f'\n                "cache": {dumps(cache.asdict())}'
+            f'\n                "cache": {cache_json}'
         )
         if serverIPAddress:
             chunk += f',\n                "serverIPAddress": {dumps(serverIPAddress)}'