default or selected kernelversion does not have config value CONFIG_CRYPTO_NULL2.
Result is shown for kernelversion 6.9.9

CONFIG_CRYPTO_NULL2 (not configurable)

modulename: crypto_null.ko

configname: CONFIG_CRYPTO_NULL2

Linux Kernel Configuration
└─>Cryptographic API
└─>Crypto core or helper
└─>CRYPTO_NULL2
In linux kernel since version 3.10 (release Date: 2013-06-30)  
source code:
crypto/crypto_null.c

depends
CONFIG_CRYPTO