MemorySegment::allocateNative should be tolerant of zero-sized allocation requests

MemorySegment::allocateNative throws IllegalArgumentException if its size parameter is <= 0. This seems to be inconsistent with the fact that Unsafe::allocate/freeMemory seems to be tolerant of that, and that you can still create zero-sized segments from e.g. direct buffers (or Java arrays).

This is discussed in more details here:
https://mail.openjdk.java.net/pipermail/panama-dev/2022-January/016071.html