CONFIG_NULL_CHAN is not available for the default architecture x86.
Result is shown for architecture um

null channel support

modulename: null.ko
configname: CONFIG_NULL_CHAN

Linux Kernel Configuration
└─> UML Character Devices
└─> null channel support

In linux kernel since version 2.6.12  

This option enables support for attaching UML consoles and serial
lines to a device similar to /dev/null. Data written to it disappears
and there is never any data to be read.

source code: