thermal: gov_power_allocator: Add missing NULL pointer check

Commit 0dc23567 ("thermal: core: Move lists of thermal instances
to trip descriptors") overlooked the case in which the Power Allocator
governor attempts to bind to a tripless thermal zone and params->trip_max
is NULL in check_power_actors().

No power actors can be found in that case, so check_power_actors() needs
to be made return 0 then to restore its previous behavior.

Fixes: 0dc23567 ("thermal: core: Move lists of thermal instances to trip descriptors")
Closes: https://lore.kernel.org/linux-pm/Z0NeGF4ryCe_b5rr@sashalap/


Reported-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/2761105.mvXUDI8C0e@rjwysocki.net