An nt_checked array with an empty initializer list should be an error

[mgrang]

Consider the following declaration of an nt_checked array:
char p nt_checked[] = {}

According to the Checked C spec section 2.4:

1. nt_checked declares an array whose last element is a null terminator. The size of the array includes the null terminator element.
2. An nt_checked array with size d converts to an nt_array_ptr with a count of d - 1 elements.

So it should be illegal to declare an nt_checked array with an empty initializer list.

[mgrang]

Fixed in #1121