Bind configuration check actor got an stacktrace if one of file included from /etc/named.conf is empty

[SandakovMM]

Actual behavior
Bind configuration got an stack trace when one of files to parse is empty.
The stack trace is:

        Check whether we are on legacy (BIOS) system and instruct Leapp to upgrade GRUB core
====> * check_se_linux
        Check SELinux status and produce decision messages for further action.
====> * check_postfix
        Check if postfix is installed, check whether configuration update is needed.
====> * check_bind
        Actor parsing BIND configuration and checking for known issues in it.
Process Process-442:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 72, in _do_run
    actor_instance.run(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 290, in run
    self.process(*args)
  File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/checkbind/actor.py", line 32, in process
    facts = iscmodel.get_facts('/etc/named.conf')
  File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/checkbind/libraries/iscmodel.py", line 81, in get_facts
    parser.walk(cfg.root_section(), find_calls, state)
  File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/libraries/isccfg.py", line 879, in walk
    for statement in it:
  File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/libraries/isccfg.py", line 223, in __next__
    statement = next(self.iter)
  File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/libraries/isccfg.py", line 181, in __next__
    val = self.parser.find_next_key(cfg, index, self.section.end)
  File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/libraries/isccfg.py", line 689, in find_next_key
    while istr[index] in self.CHAR_KEYWORD and index < end_index:
IndexError: string index out of range

To Reproduce

1. Add empty file what we will include: touch /etc/named-user-options.conf
2. Add include directive for into /etc/named.conf into options section with /etc/named-user-options.conf
3. leapp preupgrade

Expected behavior
Leapp skip empty file and proceed to work

System information (please complete the following information):

# cat /etc/os-release 
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

# uname -a
Linux ip-10-69-45-134.eu-west-1.compute.internal 3.10.0-1160.76.1.el7.x86_64 #1 SMP Wed Aug 10 16:21:17 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
# rpm -qa "*leapp*"
python2-leapp-0.14.0-1.el7.noarch
leapp-upgrade-el7toel8-deps-0.16.0-6.el7.elevate.16.noarch
leapp-upgrade-el7toel8-0.16.0-6.el7.elevate.16.noarch
leapp-data-almalinux-0.1-6.el7.noarch
leapp-deps-0.14.0-1.el7.noarch
leapp-0.14.0-1.el7.noarch

Attached logfiles

[SandakovMM]

PR request with possible fix: oamg#1124