[ErrorHandler] fix setting $trace to null in FatalError

[nicolas-grekas]

Q	A
Branch?	4.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	-
License	MIT
Doc PR	-

Spotted by @nikic in php/php-src#5620 (comment)

[derrabus]

That'll work, but since it's the second time that we have special handling for $trace === null here, I'd suggest to deal with this case at the very beginning of the method.

edit: Nevermind, $trace = null in fact does something else than $trace = [].