fs-util: Allow passing NULL path to xopenat()

We recently codified in the coding style that for openat() style APIs,
an empty path can be passed both as the empty string and as NULL, so
let's make sure we follow that style in xopenat().

src/basic/fs-util.c

@@ -1097,7 +1097,6 @@ int xopenat(int dir_fd, const char *path, int open_flags, XOpenFlags xopen_flags
         int r;
 
         assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
-        assert(path);
 
         if (isempty(path)) {
                 assert(!FLAGS_SET(open_flags, O_CREAT|O_EXCL));