PM: wakeup: Handle empty list in wakeup_sources_walk_start()

PM: wakeup: Handle empty list in wakeup_sources_walk_start()

In the case of an empty wakeup_sources list, wakeup_sources_walk_start()
will return an invalid but non-NULL address. This also affects wrappers
of the aforementioned function, like for_each_wakeup_source().

Update wakeup_sources_walk_start() to return NULL in case of an empty
list.

Fixes: b4941adb24c0 ("PM: wakeup: Add routine to help fetch wakeup source object.")
Signed-off-by: Samuel Wu <wusamuel@google.com>
[ rjw: Subject and changelog edits ]
Link: https://patch.msgid.link/20260124012133.2451708-2-wusamuel@google.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>