std::unique_ptr constexpr constructor

As shown here, std::unique_ptr has two constexpr constructors for null pointers:

constexpr unique_ptr();
constexpr unique_ptr( nullptr_t );

I have two questions for these two constructors:

(1) Why do we need two? Can't we just declare one as:

constexpr unique_ptr( nullptr_t = nullptr );

(2) Is the constexpr really useful? I tried doing this in my code but it didn't compile (g++ 6.1.0, -std=c++14):

constexpr std::unique_ptr<int> p;
// error: the type 'const std::unique_ptr<int>' of constexpr variable 'p'
// is not literal because 'std::unique_ptr<int>' has a non-trivial destructor