Try fix when script run with TamperMonkey or GreaseMonkey

[langren1353]

When useing TamperMonkey or GreaseMonkey with less.js as a @require resource, sometimes there will be an error.
I trace the code found a small problem.
At https://github.com/less/less.js/blob/master/packages/less/src/less-browser/utils.js
function addDataAttrs param tag may be null, then the script will exec failed.
A short if may fix this, will you please add the code to check if tag is null

[iChenLei]

Hi, @langren1353 PR welcome !

[langren1353]

Well, I submit one RP, hope that would work