Fix error on downloading magnetlink

[xoriole]

With PR #7842 merged, it raised a bug in creating TorrentDefNoMetainfo object while doing metainfo check. The metainfo check failed with the following stacktrace.

[tribler-gui PID:1197156] 2024-01-24 14:13:33,359 - ERROR <error_handler:100> ErrorHandler.core_error(): 'str' object has no attribute 'decode'
Traceback (most recent call last):
  File "/home/user/tribler/src/tribler/core/components/restapi/rest/rest_manager.py", line 53, in error_middleware
    response = await handler(request)
  File "/home/user/tribler/pyenv39/lib/python3.9/site-packages/aiohttp/web_middlewares.py", line 114, in impl
    return await handler(request)
  File "/home/user/tribler/src/tribler/core/components/libtorrent/restapi/torrentinfo_endpoint.py", line 131, in get_torrent_info
    metainfo = await self.download_manager.get_metainfo(infohash, timeout=60, hops=hops, url=uri)
  File "/home/user/tribler/src/tribler/core/components/libtorrent/download_manager/download_manager.py", line 520, in get_metainfo
    download = await self.start_download(tdef=tdef, config=dcfg, hidden=True, checkpoint_disabled=True)
  File "/home/user/tribler/src/tribler/core/components/libtorrent/download_manager/download_manager.py", line 653, in start_download
    atp = download.get_atp()
  File "/home/user/tribler/src/tribler/core/components/libtorrent/download_manager/download.py", line 200, in get_atp
    atp["name"] = self.tdef.get_name_as_unicode()
  File "/home/user/tribler/src/tribler/core/components/libtorrent/torrentdef.py", line 558, in get_name_as_unicode
    return self.get_name_utf8()
  File "/home/user/tribler/src/tribler/core/components/libtorrent/torrentdef.py", line 326, in get_name_utf8
    return escape_as_utf8(self.get_name(), self.get_encoding())
  File "/home/user/tribler/src/tribler/core/components/libtorrent/torrentdef.py", line 38, in escape_as_utf8
    return string.decode(encoding).encode('utf8').decode('utf8')
AttributeError: 'str' object has no attribute 'decode'

This PR addresses the issue.

[xoriole]

This is the fix. The name field is expected to be bytes.