chore: Fix acceptance tests env variable names (#170)

## Motivation

Set empty string instead of nil to "cheat" Terraform into thinking we
did provide required variable (even though we didn't).

.github/workflows/acc-tests.yml

@@ -35,8 +35,8 @@ jobs:
           cache: false
       - name: Run acceptance tests
         env:
-          TERRAFORM_NOBL9_CLIENT_ID: "${{ inputs.clientId }}"
-          TERRAFORM_NOBL9_CLIENT_SECRET: "${{ secrets.clientSecret }}"
+          NOBL9_CLIENT_ID: "${{ inputs.clientId }}"
+          NOBL9_CLIENT_SECRET: "${{ secrets.clientSecret }}"
           TERRAFORM_NOBL9_OKTA_ORG_URL: "${{ inputs.oktaOrgUrl }}"
           TERRAFORM_NOBL9_OKTA_AUTH_SERVER: "${{ inputs.oktaAuthServer }}"
           TERRAFORM_NOBL9_PROJECT: "${{ inputs.project }}"