vkr: validate dynamic arrays of size 0

Merged Chia-I Wu requested to merge olv/virglrenderer:vkr-null-array into master

This corresponds to olv/venus-protocol!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")

/cc @zzyiwei @ryanneph

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