lisa.energy_model: Fix EnergyModel building with all CPUs in one freq domain
lisa.energy_model: Fix EnergyModel building with all CPUs in one freq domain

Rather than raising an exception, taking the intersection of a set
with no other set gives back the initial set:

  set.intersection(x) == x

This made EnergyModel choke on calls with all CPUs in the same frequency
domain.

Fix that by checking the number of domains first.

credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa