vkr: validate dynamic arrays of size 0
This corresponds to https://gitlab.freedesktop.org/olv/venus-protocol/-/merge_requests/19.
The decoder gained dynamic array size validation in commit 92b083bb ("vkr: validate array sizes and string lengths") but failed for one case: fooCount != 0 and pFoo is NULL. This commit fixes that.
Fixes: 0690ad0b ("vkr: remove some manual NULL checks")