Decoding to png results in a small black image.

[TheHardew]

Describe the bug
When decoding 003.jxl, which was losslessly transcoded from a jpg, the resulting png file is only 1953 B small and all black, even though djxl didn't throw any errors.
Decoding to the original jpg file is still possible, but only because --strip was not used, otherwise it would be impossible to decode this file at all.

To Reproduce
(unpack the attached zip file to get 003.jxl)

~ $ djxl 003.jxl 003.png
JPEG XL decoder v0.3.7 [AVX2]
Read 274618 compressed bytes.
Decoded to pixels.
752 x 1080, 14.64 MP/s [14.64, 14.64], 1 reps, 8 threads.
Allocations: 476 (max bytes in use: 3.297596E+07)

~ $ djxl 003.jxl 003.jpg
JPEG XL decoder v0.3.7 [AVX2]
Read 274618 compressed bytes.
Reconstructed to JPEG.
752 x 1080, 16.33 MP/s [16.33, 16.33], 6.64 MB/s [6.64, 6.64], 1 reps, 8 threads.
Allocations: 429 (max bytes in use: 3.297534E+07)

~ $ du -b 003.*
330011	003.jpg
274618	003.jxl
1953	003.png

Expected behavior
It should be possible to decode this file to png.

Environment

• OS: Arch Linux
• Compiler version: clang 12.0.0
• CPU type: ryzen 2700x x86_64
• cjxl/djxl version string: 8193d7b

Additional context
Compiled with https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=libjxl-git and

CFLAGS="-march=native -O3 -pipe -fstack-protector-strong --param=ssp-buffer-size=4 -fno-plt -pthread -Wno-error -w"
CXXFLAGS="$CFLAGS"

003.jxl inside a zip file, otherwise github wouldn't accept it.
003.zip

[eustas]

In gray mode 0-th channel is taken for output instead of 1-st.

[eustas]

It is encoder bug =( #250
Hopefully, no data is lost (as it is possible to decode to JPEG again).

[eustas]

Should be fixed. Though requires re-encoding of monochrome JPEGs with Adobe RGB ICC.