Having audio sync issues after concatenating several mp4 with and without sound - audio

Question

I'm posting today because I have a problem when I use ffmpeg. I have a small program that creates mp4 files for me, often with sounds and sometimes without, until now I used ffmpeg to concatenate them.
But I just realized that if I have an mp4 file that doesn't contain audio then the whole audio track of the final file goes out of sync. This only happens when I have some mp4 files that don't have audio. I think it's also useful to know that the program I'm using gives me a lot of mp4 (>20) and I can't know in advance which ones have audio or not...
Here is the code I use, I saw on the forum that the mp4 format was badly managed and that it was better to use ts format for concatenation that's why I do this
for f in $(ls *.mp4); do
ffmpeg -i $f -c copy -bsf:v h264_mp4toannexb -f mpegts $f.ts
done
CONCAT=$(echo $(ls *.ts ) | sed -e "s/ /|/g")
ffmpeg -analyzeduration 2147483647 -probesize 2147483647 -i "concat:$CONCAT" -c copy output.ts
ffmpeg -analyzeduration 2147483647 -probesize 2147483647 -i output.ts -acodec copy -vcodec copy output.mp4
I noticed that when the final file goes out of sync this error appears :
Input #0, mpegts, from 'concat:video0.mp4.ts|video01.mp4.ts|video012.mp4.ts|video0123.mp4.ts|video01234.mp4.ts|video012345.mp4.ts|video0123456.mp4.ts|video01234567.mp4.ts|video012345678.mp4.ts|video0123456789.mp4.ts|video012345678910.mp4.ts|video01234567891011.mp4.ts|video0123456789101112.mp4.ts|video012345678910111213.mp4.ts|video01234567891011121314.mp4.ts|video0123456789101112131415.mp4.ts|video012345678910111213141516.mp4.ts|video01234567891011121314151617.mp4.ts|video0123456789101112131415161718.mp4.ts|video012345678910111213141516171819.mp4.ts|zzzz.mp4.ts':
Duration: 00:00:05.02, start: 1.420222, bitrate: 63089 kb/s
Program 1
Metadata:
service_name : Service01
service_provider: FFmpeg
Stream #0:0[0x100]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(tv, bt709, progressive), 640x1136, 30 fps, 240 tbr, 90k tbn, 60 tbc
Stream #0:1[0x101](und): Audio: aac (LC) ([15][0][0][0] / 0x000F), 44100 Hz, mono, fltp, 145 kb/s
Output #0, mpegts, to 'output.ts':
Metadata:
encoder : Lavf58.20.100
Stream #0:0: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(tv, bt709, progressive), 640x1136, q=2-31, 30 fps, 240 tbr, 90k tbn, 90k tbc
Stream #0:1(und): Audio: aac (LC) ([15][0][0][0] / 0x000F), 44100 Hz, mono, fltp, 145 kb/s
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
[mpegts # 0x7f9c68800000] DTS 130180 < 573000 out of order <-- HERE
[mpegts # 0x7f9c68800000] DTS 130180 < 654000 out of order <-- HERE
[mpegts # 0x7f9c68800000] DTS 130180 < 1477180 out of order <-- HERE
I've tried several different combinations of members answer on other topics but nothing worked for me.
I don't know if the problem comes from the moment I convert .mp4 files to .ts and I forget to set appropriate codec, or if it comes from the concat command ?
I'm almost sure that there is a simple way to fix this issue but unfortunately I don't have enough ffmpeg knowledge.
Thanks for your help :)
EDIT 1 :
I changed the for loop of the programme to :
for f in $(ls *.mp4); do
TEST=$(ffprobe -i $f -show_streams -select_streams a -loglevel error)
if [ "$TEST" ]; then
ffmpeg -i $f -c copy -bsf:v h264_mp4toannexb -f mpegts $f.ts
fi
done
It only makes the script to ignore the files that don't have audio, but this way it works, so maybe adding dummy audio to those files could make the job as #Gyan suggested in the comments, but how to do that ?
EDIT 2 SOLVED
I found how to add dummy audio on my silent file, that solved my problem this is my final working for loop :
for f in $(ls *.mp4); do
TEST=$(ffprobe -i $f -show_streams -select_streams a -loglevel error)
if [ "$TEST" ]; then
ffmpeg -i $f -c copy -bsf:v h264_mp4toannexb -f mpegts $f.ts
else
ffmpeg -f lavfi -i anullsrc -i $f -shortest -c:v copy -c:a aac -map 0:a -map 1:v $f.a.ts
ffmpeg -i $f.a.ts -c copy -bsf:v h264_mp4toannexb -f mpegts $f.ts
rm $f.a.ts
fi
done