[libcxx] Add checks for unique value of array<T, 0>.begin() and array<T, 0>.end()

The standard section [array.zero] requires the return value of begin()
and end() methods of a zero-sized array to be unique. Eric Fiselier
clarifies: "That unique value cannot be null, and must be properly aligned".
This patch adds checks for the first part of this clarification: unique
value returned by these methods cannot be null.

Reviewed as https://reviews.llvm.org/D55366.
Thanks to Andrey Maksimov for the patch.

llvm-svn: 348509