Support `uninitialized_array` with capacity zero

## Brief feature description

Supporting an `uninitialized_array` with a capacity of zero can be beneficial for some meta-programming corner cases. The array should provide the same interface as the `uninitialized_array` with a non-zero capacity but all methods should return either `nullptr` or dereference `nullptr`.

The issue names no implementation files or tests. Locate the existing uninitialized_array implementation and its tests, then compare the zero-capacity behavior with the non-zero interface. Done means a capacity-zero array is supported and its methods consistently return nullptr or dereference nullptr as specified.

Written by the indexing model from the issue text.