[BUG] `salt-cloud` fails for `gce.avail_sizes`

Description
Querying the available sizes in GCE using salt-cloud --list-sizes gce-test fails using SaltStack 3000.2.

Setup
/etc/salt/cloud.providers.d/google-compute-engine.conf:

gce-test:
    project: test-project
    service_account_email_address: local-dev-test-sa@test-project.iam.gserviceaccount.com
    service_account_private_key: /home/eliasp/.secrets/gce-44fda5a581cff82dba2b3b6b1b95d881c.pem
    driver: gce

Steps to Reproduce the behavior

$ salt-cloud -l debug --list-sizes gce-test
[DEBUG   ] Using cached minion ID from /etc/salt/minion_id: cloud-test
[DEBUG   ] Using cached minion ID from /etc/salt/minion_id: cloud-test
[DEBUG   ] Missing configuration file: /etc/salt/cloud.providers
[DEBUG   ] Missing configuration file: /etc/salt/cloud.providers
[DEBUG   ] Including configuration from '/etc/salt/cloud.providers.d/google-compute-engine.conf'
[DEBUG   ] Including configuration from '/etc/salt/cloud.providers.d/google-compute-engine.conf'
[DEBUG   ] Reading configuration from /etc/salt/cloud.providers.d/google-compute-engine.conf
[DEBUG   ] Reading configuration from /etc/salt/cloud.providers.d/google-compute-engine.conf
[DEBUG   ] Missing configuration file: /etc/salt/cloud.profiles
[DEBUG   ] Missing configuration file: /etc/salt/cloud.profiles
[DEBUG   ] Configuration file path: /etc/salt/master
[DEBUG   ] Configuration file path: /etc/salt/master
[WARNING ] Insecure logging configuration detected! Sensitive data may be logged.
[WARNING ] Insecure logging configuration detected! Sensitive data may be logged.
[INFO    ] salt-cloud starting
[INFO    ] salt-cloud starting
[DEBUG   ] Marking 'base64_encode' as a jinja filter
[...] removed loading of Jinja filters
[DEBUG   ] Marking 'symmetric_difference' as a jinja filter
[DEBUG   ] Marking 'symmetric_difference' as a jinja filter
[DEBUG   ] Could not LazyLoad parallels.avail_sizes: 'parallels' __virtual__ returned False
[DEBUG   ] Could not LazyLoad parallels.avail_sizes: 'parallels' __virtual__ returned False
[DEBUG   ] LazyLoaded parallels.avail_locations
[DEBUG   ] LazyLoaded parallels.avail_locations
[DEBUG   ] LazyLoaded proxmox.avail_sizes
[DEBUG   ] LazyLoaded proxmox.avail_sizes
[DEBUG   ] Starting new HTTPS connection (1): www.googleapis.com
[DEBUG   ] Starting new HTTPS connection (1): www.googleapis.com
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones HTTP/1.1" 200 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones HTTP/1.1" 200 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/regions HTTP/1.1" 200 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/regions HTTP/1.1" 200 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/aggregated/machineTypes HTTP/1.1" 200 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/aggregated/machineTypes HTTP/1.1" 200 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central1-d HTTP/1.1" 404 None
[...] removed 76 recurrences
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central1-d HTTP/1.1" 404 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-b HTTP/1.1" 404 None
[...] removed 42 recurrences
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-b HTTP/1.1" 404 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-a HTTP/1.1" 404 None
[...] removed 46 recurrences
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-a HTTP/1.1" 404 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-c HTTP/1.1" 404 None
[...] removed 42 recurrences
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-c HTTP/1.1" 404 None
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-d HTTP/1.1" 404 None
[...] removed 42 recurrences
[DEBUG   ] https://www.googleapis.com:443 "GET /compute/v1/projects/test-project/zones/us-central2-d HTTP/1.1" 404 None
[ERROR   ] Failed to get the output of 'gce.avail_sizes()': 'NoneType' object has no attribute '__dict__'
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/salt/cloud/__init__.py", line 896, in size_list
    data[alias][driver] = self.clouds[fun]()
  File "/usr/lib/python3/dist-packages/salt/cloud/clouds/gce.py", line 324, in avail_sizes
    size.extra['zone'].update(zone.__dict__)
AttributeError: 'NoneType' object has no attribute '__dict__'
[ERROR   ] Failed to get the output of 'gce.avail_sizes()': 'NoneType' object has no attribute '__dict__'
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/salt/cloud/__init__.py", line 896, in size_list
    data[alias][driver] = self.clouds[fun]()
  File "/usr/lib/python3/dist-packages/salt/cloud/clouds/gce.py", line 324, in avail_sizes
    size.extra['zone'].update(zone.__dict__)
AttributeError: 'NoneType' object has no attribute '__dict__'
[DEBUG   ] LazyLoaded nested.output
[DEBUG   ] LazyLoaded nested.output
gce-test:
    ----------

Expected behavior
I expect salt-cloud --list-sizes gce-test to return a list of available VM sizes + no Traceback/AttributeError.

Versions Report

Salt Version:
            Salt: 3000.2

Dependency Versions:
 Apache Libcloud: 2.2.1
            cffi: Not Installed
        cherrypy: Not Installed
        dateutil: 2.6.1
       docker-py: Not Installed
           gitdb: Not Installed
       gitpython: Not Installed
          Jinja2: 2.10
         libgit2: Not Installed
        M2Crypto: Not Installed
            Mako: 1.0.7
    msgpack-pure: Not Installed
  msgpack-python: 0.6.2
    mysql-python: Not Installed
       pycparser: Not Installed
        pycrypto: 2.6.1
    pycryptodome: Not Installed
          pygit2: Not Installed
          Python: 3.6.9 (default, Apr 18 2020, 01:56:04)
    python-gnupg: 0.4.1
          PyYAML: 3.12
           PyZMQ: 16.0.2
           smmap: Not Installed
         timelib: Not Installed
         Tornado: 4.5.3
             ZMQ: 4.2.5

System Versions:
            dist: Ubuntu 18.04 bionic
          locale: UTF-8
         machine: x86_64
         release: 4.15.0-76-generic
          system: Linux
         version: Ubuntu 18.04 bionic

Additional context
Possibly related: #56862
salt-cloud --list-locations gce-test works just fine, so it's not the provider configuration which is broken.

Start in salt/cloud/clouds/gce.py at avail_sizes, especially the failing zone update on line 324, and trace how salt/cloud/init.py invokes it through size_list. Reproduce with salt-cloud --list-sizes gce-test using the reported provider configuration, then verify that available VM sizes are returned without the NoneType AttributeError.