Fix assertion on empty array case

[OlivierNicole]

Fixes #13939.

For consistency, I added an assertion in caml_floatarray_unsafe_set where it was absent before.

[gasche]

Two comments:

• Another approach would have been to move the assertion after the bound check. I like your current approach a bit more because it is explicit about the intended dynamic type of the input.

• I have mixed feelings about silencing an assert failure (in debug mode) when calling unsafe_{get,set} on empty arrays, which is undefined behavior. On the other hand, by this reasoning we could also assert that the access is in-bounds in general, and we don't do this, so allowing accesses on 0-sized atoms is arguably consistent.

[gasche]

I'm not sure whether or not we wanted a Changes entry, but I was tempted to click "merge" so I added the label to skip it. Please feel free to just direct-push one if you forgot but think that one is useful.

[OlivierNicole]

I just force-pushed an amended commit with the changelog updated.