Make container classes in SkTemplates.h more consistent

Make container classes in SkTemplates.h more consistent

Ensures that ".get()" always returns null when a container is empty.
Also ensures consistent assert behavior for array counts.

There are still differences in that the malloc variants take a size_t
and the arrays take an int, and that SkAutoSTMalloc defaults to the
stack-allocated buffer wheras the other containers default to null.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2084213003

Review-Url: https://codereview.chromium.org/2084213003