How to avoid UB in this code?

append-ub.c

//
// Adding zero to a null-pointer is undefined behavior!
//
// In the following code, String a is dynamically sized array of chars.
// UB is triggerred when appending an empty string to an freshly initialized String.
// See line 50.
//
// What is the recommended approach to avoid this error?
//
// Compile and run with:
//   clang -fsanitize=undefined append-ub.c
//   ./a.out
//

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

typedef struct {
    char *data;
    int size;
    int capacity;
} String;

void reserve(String *a, int required) {
    if (a->size + required <= a->capacity) {
        return;
    }
    a->capacity = a->size + required;
    a->data = realloc(a->data, a->capacity);
}

void append(String *a, char *s) {
    int n = strlen(s);
    reserve(a, n);
    memcpy(a->data + a->size, s, n);
    a->size += n;
}

void show(String *a) {
    printf("STR: '");
    fwrite(a->data, 1, a->size, stdout);
    printf("'\n");
}

int main() {
    String a;
    memset(&a, 0, sizeof(a));
    show(&a);
    append(&a, "");
    show(&a);
    free(a.data);
    return 0;
}