The only value it will ever have is NULL (but calling through a NULL function pointer is UB so it can instead call system() :)
edit: I was wrong, see below.
If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:
— if it has pointer type, it is initialized to a null pointer;
From 3.6.2:
>Variables with static storage duration (3.7.1) or thread storage duration (3.7.2) shall be zero-initialized (8.5) before any other initialization takes place
and from 8.5:
>To zero-initialize an object or reference of type T means
>--if T is a scalar type (3.9), the object is initialized to the value obtained by converting the integer literal 0 (zero) to T
and from 3.9:
>Arithmetic types (3.9.1), enumeration types, pointer types, pointer to member types (3.9.2), std::nullptr_- t, and cv-qualified versions of these types (3.9.3) are collectively called scalar types