failing to convert ass to srt subs

[username0422]

Summary of the bug: Converting ass to srt subs produces an empty output.
How to reproduce:

% ffmpeg -i input.ass output.srt
ffmpeg 4.4.1 OR ffmpeg version 4.2.4
built on 20211217

ffmpeg version N-59934-g68457c1e85-static ​https://johnvansickle.com/ffmpeg/ Copyright (c) 2000-2021 the FFmpeg developers

built with gcc 8 (Debian 8.3.0-6)
configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libdav1d --enable-libxvid --enable-libzvbi --enable-libzimg
libavutil 57. 11.100 / 57. 11.100
libavcodec 59. 14.100 / 59. 14.100
libavformat 59. 10.100 / 59. 10.100
libavdevice 59. 0.101 / 59. 0.101
libavfilter 8. 20.100 / 8. 20.100
libswscale 6. 1.101 / 6. 1.101
libswresample 4. 0.100 / 4. 0.100
libpostproc 56. 0.100 / 56. 0.100

Splitting the commandline.
Reading option '-v' ... matched as option 'v' (set logging level) with argument 'debug'.
Reading option '-i' ... matched as input url with argument 'input.ass'.
Reading option 'output.srt' ... matched as output url.
Finished splitting the commandline.
Parsing a group of options: global .
Applying option v (set logging level) with argument debug.
Successfully parsed a group of options.
Parsing a group of options: input url input.ass.
Successfully parsed a group of options.
Opening an input file: input.ass.
[NULL @ 0x7094c00] Opening 'input.ass' for reading
[file @ 0x7095400] Setting default whitelist 'file,crypto,data'
[ass @ 0x7094c00] Format ass probed with size=2048 and score=100
[ass @ 0x7094c00] Before avformat_find_stream_info() pos: 94758 bytes read:94758 seeks:0 nb_streams:1
[ass @ 0x7094c00] All info found
[ass @ 0x7094c00] After avformat_find_stream_info() pos: 94758 bytes read:94758 seeks:0 frames:0
Input #0, ass, from 'input.ass':

Duration: N/A, bitrate: N/A
Stream #0:0, 0, 1/100: Subtitle: ass

Successfully opened the file.
Parsing a group of options: output url output.srt.
Successfully parsed a group of options.
Opening an output file: output.srt.
File 'output.srt' already exists. Overwrite? [y/N] y
[file @ 0x709aa00] Setting default whitelist 'file,crypto,data'
Successfully opened the file.
Output #0, srt, to 'output.srt':

Metadata:

encoder : Lavf59.10.100

Stream #0:0, 0, 1/1000: Subtitle: subrip

Metadata:

encoder : Lavc59.14.100 srt

Stream mapping:

Stream #0:0 -> #0:0 (ass (ssa) -> subrip (srt))

Press [q] to stop, ? for help
cur_dts is invalid st:0 (0) [init:1 i_done:0 finish:0] (this is harmless if it occurs once at the start per stream)
No more output streams to write to, finishing.peed= 0x
size= 0kB time=00:00:00.00 bitrate=N/A speed= 0x
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Input file #0 (input.ass):

Input stream #0:0 (subtitle): 0 packets read (0 bytes); 0 frames decoded;
Total: 0 packets (0 bytes) demuxed

Output file #0 (output.srt):

Output stream #0:0 (subtitle): 0 frames encoded; 0 packets muxed (0 bytes);
Total: 0 packets (0 bytes) muxed

Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)
[AVIOContext @ 0x7096900] Statistics: 0 bytes written, 0 seeks, 0 writeouts
0 frames successfully decoded, 0 decoding errors
[AVIOContext @ 0x709d7c0] Statistics: 94758 bytes read, 0 seeks

[username0422]

sample ass file

[Carl Eugen Hoyos]

Our ass parser does not like the empty layer fields, I don't know if the input file is valid, patch sent.