Describe the bug
It should be impossible to instantiate a PVSystem with no Arrays. Currently this is possible via PVSystem(arrays=[]).

To Reproduce
Steps to reproduce the behavior:

from pvlib import pvsystem
pvsystem.PVSystem(arrays=[])

results in this PVSystem:

PVSystem:
  name: None
  inverter: None

Expected behavior
A ValueError should be raised indicating that a PVSystem must have at least one Array and suggesting that a system with an arbitrary default array can be constructed by passing arrays=None or not passing the arrays parameter at all.

Versions:

• pvlib.__version__: 0.8.1+