if you pass futimesat or utimensat a NULL path, the functions will operate on the fd as given.  in which case it can be either a fd for a directory or a file.

for example, the two calls to futimesat() below will do the same thing:
#include <fcntl.h>
#include <stdio.h>
int main() {
    int fd = open("f", O_CREAT|O_RDWR, 0777);
    printf("futimesat = %i\n", futimesat(fd, NULL, NULL));
    printf("futimesat = %i\n", futimesat(AT_FDCWD, "f", NULL));
    return 0;
}

same behavior can be observed with utimensat(), but it seems only at the Linux syscall level.  the POSIX/C library level will reject a NULL filename with errno==EINVAL, but the Linux kernel will treat the dirfd as an fd.  funny enough, while glibc rejects pathname==NULL at its ABI, it relies on the Linux ABI to accept pathname==NULL while implementing other functions.