CONFIG_NULL_CHAN is not available for thedefaultarchitecture 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: