Code guidelines

Witch style (F1, F2 ..) better represents the guidelines of your code?
//The function f never returns nullptr
int * f();
void F1() {
    int* p = f();
    if (p == nullptr) {
       assert(false);
       return;
    }
    // using *p ...
}
void F2()
{
    int* p = f();
    assert(p != nullptr);
    // using *p ...
}
void F3()
{
    int* p = f();
    // using *p ...
}
void F4()
{
    int* p = f();
    if (p) {
     // using *p ...
    }
}
The same question but for struct members
struct user {
   //Name is required, cannot be null.
   char * name;
};
void F1(struct user* user) {
    if (user->name == nullptr) {
       assert(false);
       return;
    }
    // using user->name ...
}
void F2(struct user* user) {
    assert(user->name != nullptr);
    // using user->name ...
}
void F3(struct user* user) {
    // using user->name ...
}
void F4(struct user* user) {
    if (user->name) {
      // using user->name ...
    }
}