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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_... 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 ```