system/nxinit: skip actions without commands

[JianyuWang0623]

Note: Please adhere to Contributing Guidelines.

Summary

An on section in the rc file can end up with no commands: the section line is
accepted on its own, and a build that runs the rc file through the C
preprocessor can have every command inside a section compiled out. add_ready()
queues such an action anyway, and init_action_run_command() then takes the head
of its empty command list, which list_peek_head_type() reports as NULL, and
dereferences it, so init dies at boot.

Skip actions that have no commands. A cmocka regression test follows.

Impact

An rc file containing an empty on section no longer crashes init at boot; the
section is simply never queued. Nothing else changes: no new Kconfig symbol, no
new dependency, and the test code builds only under the existing
CONFIG_SYSTEM_NXINIT_TEST.

Testing

Host Ubuntu 22.04.5 x86_64, gcc 13.4.0; target sim:nsh, nuttx at 284d4d4e,
nuttx-apps at 27ecdd7.

On top of sim:nsh:

CONFIG_EXPERIMENTAL=y
CONFIG_LIBC_EXECFUNCS=y
CONFIG_SCHED_CHILD_STATUS=y
CONFIG_SYSTEM_NXINIT=y
CONFIG_ALLOW_MIT_COMPONENTS=y
CONFIG_LIBC_REGEX=y
CONFIG_TESTING_CMOCKA=y
CONFIG_SYSTEM_NXINIT_TEST=y

Before:

[  FAILED  ] test_nxinit_action_empty_skipped

and with the test's ready-queue assertion dropped so that
init_action_run_command() is reached:

timeout: the monitored command dumped core

After:

[==========] nxinit_tests: 20 test(s) run.
[  PASSED  ] 20 test(s).

[cederom]

Good catch thank you @JianyuWang0623 :-)