Address permadiff in vm-instance module

[tpdownes]

When used to provision A2 machine types, using the empty list as the default value of guest_accelerator causes a permadiff because the Compute Engine API reports the A2's A100 GPU card and we have chosen an explicit value. If, instead, we use a null value for guest_accelerator, Terraform treats the difference as not requiring action.

Submission Checklist

• Have you installed and run this change against pre-commit? (pre-commit install)
• Are all tests passing? (make tests)
• Have you written unit tests to cover this change?
• Is unit test coverage still above 80%?
• Have you updated all applicable documentation?
• Have you followed the guidelines in our Contributing document?

[heyealex]

Ran a quick test to be sure, and everything looks good.