Fix ValidationError that occurred when requesting file information for specific video files

[evgkirov]

API spec says that these fields are nullable, but pyuploadcare expected them to have values:

• VideoStreamInfo#bitrate
• VideoStreamInfo#codec
• VideoInfo#duration
• VideoInfo#bitrate

That sometimes led to errors like this:

pydantic.error_wrappers.ValidationError: 1 validation error for ParsingModel[FileListResponse]
__root__ -> results -> 79 -> content_info -> video -> video -> 0 -> bitrate
none is not an allowed value (type=type_error.none.not_allowed)