HTTP/3: avoid undefined behaviour with empty encoder field values

A zero length field value in encoder stream instructions left the value
data pointer NULL, passed then to ngx_memcpy() in ngx_http_v3_insert().
For the reasons outlined in b0a4d0fb8, it is now set to an empty string.

src/http/v3/ngx_http_v3_parse.c

@@ -1543,7 +1543,7 @@ ngx_http_v3_parse_field_inr(ngx_connection_t *c,
 
             st->literal.length = st->pint.value;
             if (st->literal.length == 0) {
-                st->value.len = 0;
+                ngx_str_set(&st->value, "");
                 goto done;
             }
 
@@ -1667,7 +1667,7 @@ ngx_http_v3_parse_field_iln(ngx_connection_t *c,
 
             st->literal.length = st->pint.value;
             if (st->literal.length == 0) {
-                st->value.len = 0;
+                ngx_str_set(&st->value, "");
                 goto done;
             }