progresshook on fragment.py: Inconsistent behavior on total_bytes

[zhuyifei1999]

In the documentation of progress_hooks, it's said that that if total_bytes is unknown, the value would be None:

    progress_hooks:    A list of functions that get called on download
                       progress, with a dictionary with the entries
                       * status: One of "downloading", "error", or "finished".
                                 Check this first and ignore unknown values.
                       If status is one of "downloading", or "finished", the
                       following properties may also be present:
...
                       * total_bytes: Size of the whole file, None if unknown

However this is not the case for fragment.py, where the key simply doesn't exist:

        # This dict stores the download progress, it's updated by the progress
        # hook
        state = {
            'status': 'downloading',
            'downloaded_bytes': 0,
            'frag_index': 0,
            'frag_count': total_frags,
            'filename': ctx['filename'],
            'tmpfilename': ctx['tmpfilename'],
        }

This has caused downstream issues such as KeyError: 'total_bytes'

[dstftw]

the following properties may also be present:

None of these fields are guaranteed to be present at all. total_bytes is unknown beforehand for fragment based content. total_bytes_estimate may be available for non live streams.

[zhuyifei1999]

It may be None for unknown values, as explained so in docs, or the documentation itself is not consistent with the behavior.

[dstftw]

It may be present or may not. If present it may contain a value or None. Unless mentioned otherwise, None is equivalent to absence of information.