Werk #7770: Fix "Periodic service discovery" disabling rules breaking config

In 1.6.0b1 a rule in the ruleset "Periodic service discovery" that is configured with the option "Do not perform service discovery check" resulted in the following exception when updating the core config (e.g. with cmk -U):

OMD[testsite]:~$ cmk --debug -Uv
Generating configuration for core (type cmc)...
time period '24X7': 2 time points
Configuration Error: 'NoneType' object has no attribute '__getitem__'
Traceback (most recent call last):
File "/omd/sites/testsite/bin/cmk", line 94, in
exit_status = modes.call(mode_name, mode_args, opts, args)
File "/omd/sites/testsite/lib/python/cmk_base/modes/__init__.py", line 72, in call
return mode.handler_function(*handler_args)
File "/omd/sites/testsite/lib/python/cmk_base/modes/check_mk.py", line 1106, in mode_update
do_update(create_core(options), with_precompile=True)
File "/omd/sites/testsite/lib/python/cmk_base/core_config.py", line 266, in do_update
do_create_config(core, with_agents=with_precompile)
File "/omd/sites/testsite/lib/python/cmk_base/core_config.py", line 219, in do_create_config
create_core_config(core)
File "/omd/sites/testsite/lib/python/cmk_base/core_config.py", line 235, in create_core_config
core.create_config()
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 63, in create_config
_create_config_hook(self._cmc_file)
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 123, in _create_config_hook
hosts_config = _measure_time(cmc_all_hosts)
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 156, in _measure_time
result = func(*args, **kwargs)
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 457, in cmc_all_hosts
cmc_hosts = CMCHosts(config_cache.all_active_hosts(), CMCHostConfig)
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 501, in __init__
self._compute(hostnames, host_class)
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 513, in _compute
host_config = host_class(hostname)
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 966, in __init__
self._compute()
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 984, in _compute
self._cmc_services()
File "/omd/sites/testsite/lib/python/cmk_base/cee/core_cmc.py", line 1259, in _cmc_services
if disc_check_params["check_interval"] \
TypeError: 'NoneType' object has no attribute '__getitem__'

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