### Description

ACC module [cdr_extra modparam](http://kamailio.org/docs/modules/devel/modules/acc.html#acc.p.cdr_extra) allows saving additional custom values in cdr entries using dlg_vars.

When you define a new value to be saved:

`modparam("acc", "cdr_extra", "c1=$dlg_var(caller)")`

and you set a $dlg_var(c1) in the logic, everything works as expected.

But if you set it to $null or you don't even define it, it is saved as '' (empty string).

Would be nice that setting to $null would store the underlying null value (NULL in SQL database) and not defining it would lead to not even including it in the final INSERT?

### Troubleshooting

#### Reproduction

Define a new value to be saved:

`modparam("acc", "cdr_extra", "c1=$dlg_var(caller)")`

And in the kamailio.cfg logic:

- If you set $dlg_var(c1) to any string ('', 'foo', '2'), it is saved as expected ('', 'foo', '2').

- If you set $dlg_var(c1) to an integer different from 0 (1, 2, 3), it is saved as a string containing that number ('1', '2', '3'). This seems normal as dlg_vars can only store strings.

- If you set $dlg_var(c1) to 0, it is saved as an empty string (''). Seems normal too.

- If you set $dlg_var(c1) to $null, it is saved as an empty string ('').

- If you don't define $dlg_var(c1) at all, it is saved as an empty string ('').

#### Debugging Data

Both [_extra2strar_](https://github.com/kamailio/kamailio/blob/e101f21c37238862a474c50218d11a475d...) and [_extra2strar_dlg_only_](https://github.com/kamailio/kamailio/blob/e101f21c37238862a474c50218d11a475d...) functions defined in [_acc_extra.c_](https://github.com/kamailio/kamailio/blob/master/src/modules/acc/acc_extra.c) set _cdr_type_array_ entry to TYPE_NULL, but that array is not used in subsequent [for loop](https://github.com/kamailio/kamailio/blob/e101f21c37238862a474c50218d11a475d...).

### Possible Solutions

Modify the logic in the [for loop](https://github.com/kamailio/kamailio/blob/e101f21c37238862a474c50218d11a475d...) so that it invokes VAL_NULL macro for null values using _cdr_type_array_.

With this change both $null values and undefined values would use underlying NULL value. I could try to make further changes to avoid the key in the insert for undefined values instead of setting it to NULL.

### Additional Information

* **Kamailio Version** - output of `kamailio -v`

``` version: kamailio 4.4.6 (x86_64/linux) 0a8379-dirty flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: 0a8379 -dirty compiled on 12:23:36 Dec 20 2017 with gcc 6.3.0

```

* **Operating System**:

``` Linux 4.9.0-3-amd64 #1 SMP Debian 4.9.30-2+deb9u3 (2017-08-06) x86_64 GNU/Linux ```

Is there any reason I missed for not doing this change ([this comment](https://github.com/kamailio/kamailio/blob/e101f21c37238862a474c50218d11a475d...) makes me think so)? If not, I can try to provide a PR with these changes.

Regards,