Skip terraform region validation for resources without region

[turkenh]

Description of your changes

Some resources, like iam group, do not have a notion of region hence we have no region in their schema. However, terraform still attempts validating region in provider config and does not like both empty string or not setting it at all. We need to skip region validation in this case.

Fixes #112

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Create a user using the example Delete user

[github-actions[bot]]

Successfully created backport PR #116 for release-0.2.

[luebken]

LGTM