Generating Random Numbers in C and writing it to a text file

So I've created a project that reads numbers from a text file and draws an isometric projection of it, but now I'm trying to create a program that generates numbers from 0-9 and writes them in the document. This is what my code looks like, but the document remains empty. I'm under the assumption that the error is either in my rand() function usage, or when I convert the integers to characters.

Thank you in advance for all the input, and I apologize if it's just an operator error. I'm pretty new to this stuff:

#include <stdio.h>
#include <stdlib.h>

int     main(void)
{
    char    str[10];
    FILE    *fptr;
    int     i;
    int     num;
    char    num2;
    i = 0;

    fptr = fopen("map.fdf", "w");
    if (fptr == NULL)
    {
        printf("ERROR Creating File!");
        exit(1);
    }
    while (str[i] != '\0')
    {
        num = rand() % 10;
        num2 = num + '0';
        str[i] = num2;
        i += 1;
    }
    puts(str);
    fprintf(fptr,"%s", str);
    fclose(fptr);
    return (0);
}