You cannot. 0 is the value you would want to use. You essentially need to reserve a specific value and treat that one as a "NULL". With vJASS, struct(0) is used as the NULL value.
darthfett already explained it, it simply is not possible, the reason why some variables can be set to null and others (numbers) can not, is because other variables contain a handle (a number generated by wc3) which is never 1, so setting that variable to null actually just sets it to 0 which wc3 treats as a null handle, integers/reals cant be set to null because well... 0 is null, the only way to get an integer/real variable that equals nothing is with locals:
[ljass]local integer i[/ljass] local variables like this that do not have an initialized value have no value, but they cause errors if you try to reference them like an integer until they are set, ie:
[ljass]local integer i = 0[/ljass] is an initialized variable
JASS:
functionExampletakesnothingreturnsnothinglocalintegeri=0seti=i+1//worksendfunctionfunctionExampletakesnothingreturnsnothinglocalintegeriseti=i+1//doesnt work, cuz i = ??? so set i = ??? + 1 = ???... it cant do itendfunction
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory