[sr-dev] [kamailio/kamailio] s.select transformation not returning null for nonexistent indexes (#2128)

[pwakano]

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### Description
When trying to iterate over a semi-colon separated string list, the s.select transformation seems to be returning empty string instead of null, so the suggested idea from here does not work -> https://www.kamailio.org/wiki/tutorials/faq/main#how_to_iterate_through_the_ite 
It is expected that the access to an invalid index would return null.

### Troubleshooting
avp and var variables were tried and both had the same result.

### Reproduction
Original piece of code is below. The while loop will run until the max_while_loops is reached. 
The "DEBUG" log line will print the loop iterator value and an empty string for nonexistent values.
```
        $var(allowed_headers) = "X-header1; X-header2;";
        if (not_empty("$var(allowed_headers)")) {
                xlog("L_NOTICE","Forwarding headers: $avp(allowed_headers)\n");
                $var(i) = 0;
                while($(var(allowed_headers){s.select,$var(i),;}) != $null)
                {
                        xlog("L_NOTICE","DEBUG: $var(i) - $(var(allowed_headers){s.select,$var(i),;})\n");
                        $var(hdr) = $(var(allowed_headers){s.select,$var(i),;}{s.trim});
                        if (not_empty("$var(hdr)") &&
                                not_empty("$(hdr($var(hdr)))")) {
                                xlog("L_NOTICE","Header: $var(hdr): $(hdr($var(hdr))) allowed\n");
                                append_hf("$var(hdr): $(hdr($var(hdr)))\r\n");
                        }
                        $var(i) = $var(i) + 1;
                }
        }
```

It is also possible to simulate it by simply doing this:
```
        $var(allowed_headers) = "X-header1; X-header2;";
        if ($(var(allowed_headers){s.select,100,;}) != $null) {
                xlog("L_ERR","THIS IS WRONG :(");
        }
```

### Possible Solutions
Other while conditions can be done. but this is the most elegant and is not working.... For example, checking with "not_empty" works, but then we can't actually have empty values in the semi-colon separated list because it would stop the loop and entries after that would not be reached....

### Additional Information

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

```
# kamailio -v
version: kamailio 5.2.2 (x86_64/linux) 67f967
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, 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_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: 67f967 
compiled on 11:40:41 Mar 11 2019 with gcc 4.8.5
```

* **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`)
-->

```
# uname -a
Linux voice-conn-kamailio-1 3.10.0-514.21.2.el7.x86_64 #1 SMP Tue Jun 20 12:24:47 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
```


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