Crash when calling refresh_devices(expand_vsys=False)

Describe the bug

After establishing an initial connection to Panorama, calling the refresh_devices() function with expand_vsys set to False throws an AttributeError exception.

Expected behavior

The devices should be returned from Panorama.

Current behavior

An Exception is thrown.

Possible solution

Unsure at this point. Will update later if I'm able to find one.

Current workaround is to leave it as its default value of True: expand_vsys=True

Steps to reproduce

1. Run this code snippet, substituting in the correct hostname, username, and password:

from panos.panorama import Panorama
pano = Panorama(hostname="foo", api_username="admin", api_password="admin")
pano.refresh_devices(expand_vsys=False)

Screenshots

Hostname, credentials, and device group names changed to example values.

>>> from panos.panorama import Panorama
>>> pano = Panorama(hostname="foo", api_username="admin", api_password="admin")
>>> pano.refresh_devices(expand_vsys=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/matt/Library/Caches/pypoetry/virtualenvs/nautobot-plugin-chatops-panorama-B7XIf4lc-py3.9/lib/python3.9/site-packages/panos/panorama.py", line 640, in refresh_devices
    firewall_instances = tmp_fw.refreshall_from_xml(
  File "/Users/matt/Library/Caches/pypoetry/virtualenvs/nautobot-plugin-chatops-panorama-B7XIf4lc-py3.9/lib/python3.9/site-packages/panos/firewall.py", line 378, in refreshall_from_xml
    fw._set_version_and_version_info(entry.findtext("sw-version"))
  File "/Users/matt/Library/Caches/pypoetry/virtualenvs/nautobot-plugin-chatops-panorama-B7XIf4lc-py3.9/lib/python3.9/site-packages/panos/base.py", line 4027, in _set_version_and_version_info
    tokens = self.version.split(".")[:3]
AttributeError: 'NoneType' object has no attribute 'split'
>>> pano.refresh_devices()
[<DeviceGroup Foobar1 0x105cab230>, <DeviceGroup Foobar2 0x105cabee3>]
>>> 

Context

I am trying to pull a list of Palo Alto firewall devices, but am unable to use this optional argument.

Your Environment

• Version used: 1.4.0
• Environment name and version (e.g. Chrome 59, node.js 5.4, python 3.7.3): Python 3.9.6, Panorama 10.1.0
• Operating System and version (desktop or mobile): OSX 11.4
• Link to your project: n/a (internal project)

Start in panos/panorama.py at refresh_devices(), then follow the traceback through panos/firewall.py refreshall_from_xml() and panos/base.py _set_version_and_version_info(). Reproduce against the stated Panorama and expand_vsys=False path; done means the call returns the devices without the AttributeError while the default behavior remains intact.