Why you don't have to return value from recursive function to get right returning value in C?

Question

Here is a simple recursive function that emulates a while loop. In function, I return value in the end step but in the backward step of recursion I don't return value... Function surprisingly does not return NULL value but instead returns the value as it supposed to. Can someone please explain why C permits this?

#include <stdio.h>

int while_recursion(int c) {
    c++;
    if (c < 10) {
        while_recursion(c);  // <---------- NOTICE THAT I DON"T RETURN VALUE IN BACKWARD PASS
    } else
        return c;
}

int main() {
    printf("%i\n", while_recursion(0));
    return 0;
}