Fix test failure introduced by previous commit

jv.c

@@ -473,10 +473,10 @@ static jv jvp_string_copy_replace_bad(const char* data, uint32_t length) {
 
 /* Assumes valid UTF8 */
 static jv jvp_string_new(const char* data, uint32_t length) {
-  assert(data);
   jvp_string* s = jvp_string_alloc(length);
   s->length_hashed = length << 1;
-  memcpy(s->data, data, length);
+  if (data != NULL)
+    memcpy(s->data, data, length);
   s->data[length] = 0;
   jv r = {JV_KIND_STRING, 0, 0, 0, {&s->refcnt}};
   return r;