How to POSIX-ly ignore "warning: command substitution: ignored null byte in input"?

Today, I was working with my Raspberry Pi 4 with Debian 12 Bookworm, and I found it odd, that some system text files, namely I needed these two:

/sys/firmware/devicetree/base/model
/sys/firmware/devicetree/base/serial-number

were, upon reading and storing in a variable in bash like this:

rpi_model_name=$(cat /sys/firmware/devicetree/base/model)
rpi_serial_number=$(cat /sys/firmware/devicetree/base/serial-number)

producing the following warning

warning: command substitution: ignored null byte in input

I tried to get rid of the warning by redirecting stderr to /dev/null; for instance:

rpi_model_name=$(cat /sys/firmware/devicetree/base/model 2>/dev/null)

or

rpi_model_name=$(cat /sys/firmware/devicetree/base/model) 2>/dev/null

but we still get that annoying warning:

-bash: warning: command substitution: ignored null byte in input

I just have two shells installed, bash and dash. While dash behaves _normal_ as in without that warning; bash appears to be more _mouthful_ about this. So, at this point, I can't be sure if other shells _spit_ this or some similar warning too. My goal is to suppress this warning for all shells. I do not care about its origin or value, I just want it gone. The solution must be written POSIX-ly for portability.