azuredevops_build_folder_permissions - Avoid potential null pointer

[xuzhang3]

All Submissions:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My code follows the code style of this project.
• I ran lint checks locally prior to submission.
• Have you checked to ensure there aren't other open PRs for the same update/change?

What about the current behavior has changed?

Checks for the existence of the folder, and returns error message if the folder not found.

Issue Number: #710

=== RUN   TestAccBuildDefinitionPermissions_SetPermissions
=== PAUSE TestAccBuildDefinitionPermissions_SetPermissions
=== RUN   TestAccBuildDefinitionPermissions_UpdatePermissions
=== PAUSE TestAccBuildDefinitionPermissions_UpdatePermissions
=== CONT  TestAccBuildDefinitionPermissions_SetPermissions
=== CONT  TestAccBuildDefinitionPermissions_UpdatePermissions
--- PASS: TestAccBuildDefinitionPermissions_SetPermissions (54.82s)
--- PASS: TestAccBuildDefinitionPermissions_UpdatePermissions (74.52s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        81.129s

Does this introduce a change to go.mod, go.sum or vendor/?

• Yes
• No

Does this introduce a breaking change?

• Yes
• No

Any relevant logs, error output, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Other information