Update 5.null.md

[ZGuQing]

a XOR b

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jamieliu1023]

@ZGuQing Thanks for your contribution to the Nebula Graph community! This is Jamie with Nebula and I'd like to email you the Nebula Contributor certificate and ship you a mug to mark this special moment. Could you please kindly reach me via jamie.liu(at)vesoft.com? Again, thanks for being a part of the Nebula community!