### Description Intermittently the following error is observed when checking if given header is present in SIP INVITE: SystemError: <built-in function is_present> returned a result with an error set

The complete stack trace is (some names redacted): 11(28) ERROR: {1 195 INVITE nMHZDN.10nOIDt4ldOwz5cqz0XnMepA9} app_python3 [python_support.c:156]: python_handle_exception(): apy_exec: modify_headers((null)): Unhandled exception in the Python code: TypeError: an integer is required (got type NoneType)

The above exception was the direct cause of the following exception:

Traceback (most recent call last): File "/etc/kamailio/kamailio.py", line 25, in modify_headers self.modify_header("SomeHeader") File "/etc/kamailio/kamailio.py", line 29, in modify_header present = KSR.hdr.is_present(header_name) SystemError: <built-in function is_present> returned a result with an error set

### Additional Information

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

``` version: kamailio 5.4.6 (x86_64/linux) 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: unknown compiled with gcc 8.3.0 ```

* **Operating System**:

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``` Linux kamailio-5c7484c74d-vhhr8 5.4.0-1056-azure #58~18.04.1-Ubuntu SMP Wed Jul 28 23:14:18 UTC 2021 x86_64 GNU/Linux ```