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### Description

<!-- Explain what you did, what you expected to happen, and what actually happened. --> Just as in #1378, allowing nullable entries for SQL would be appreciated in acc `db_extra` and acc `multi_leg_info`.

### Expected behavior Empty/NULL values should be inserted with `NULL`. For example, I can't create a `timestamp without time zone` to be used with multi-leg-accounting as follows. ``` modparam("acc", "multi_leg_info", "leg_dst_user=$avp(leg_dst_user);" "leg_dst_domain=$avp(leg_dst_domain);" "leg_sip_code=$avp(leg_sip_code);" "leg_time=$avp(leg_time);" "leg_to_tag=$avp(leg_to_tag)")

$avp(leg_time)=$timef(%F %T)

```

#### Actual observed behavior Emplty/NULL values are inserted with `''` which fails as `''` is invalid input for a `timestamp without time zone` column.

#### Log Messages

<!-- Check the syslog file and if there are relevant log messages printed by Kamailio, add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site). -->

``` db_postgres [km_dbase.c:266]: db_postgres_submit_query(): 0x7f26f7fb6cc8 PQsendQuery Error: ERROR: invalid input syntax for type timestamp: "" ```

#### SIP Traffic

<!-- If the issue is exposed by processing specific SIP messages, grab them with ngrep or save in a pcap file, then add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site). -->

``` (paste your sip traffic here) ```

### Possible Solutions

<!-- If you found a solution or workaround for the issue, describe it. Ideally, provide a pull request with a improvement. -->

### Additional Information

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

``` ersion: kamailio 5.4.1-8.gitd43872c11e.fc32.2 (x86_64/linux) 09fd6a flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, 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, TLS_PTHREAD_MUTEX_SHARED ADAPTIVE_WAIT_LOOPS 1024, MAX_RECV_BUFFER_SIZE 262144, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: 09fd6a compiled on 00:00:00 Oct 22 2020 with gcc 10.2.1 ```

* **Operating System**:

<!-- Details about the operating system, the type: Linux (e.g.,: Debian 8.4, Ubuntu 16.04, CentOS 7.1, ...), MacOS, xBSD, Solaris, ...; Kernel details (output of `uname -a`) -->

``` Linux host 5.8.15-201.fc32.x86_64 #1 SMP Thu Oct 15 15:56:44 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux ```