White notification LED not working in Nougat
Go to Settings-> Notifications-> Tap on settings symbol on top. 

Under Lights check whether notification light pulse is enabled or not. 
Also you can set custom notification colors, 

Goto Notification light section(down), Inside that, under General section, check whether notification light enabled or not.

Under Advanced section, tap on use custom values, which enables a + on top-right, click on it, select WhatsApp and enable a custom value for notification light.
(These settings from Nougat 7.1.2 )