[Enhance] - azuredevops_serviceendpoint_aws - Add nil check and update test case

[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?

Update test case and add nil check

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

[xuzhang3]

=== RUN   TestAccServiceEndpointAws_basic
=== PAUSE TestAccServiceEndpointAws_basic
=== RUN   TestAccServiceEndpointAws_complete
=== PAUSE TestAccServiceEndpointAws_complete
=== RUN   TestAccServiceEndpointAws_update
=== PAUSE TestAccServiceEndpointAws_update
=== RUN   TestAccServiceEndpointAws_oidc
=== PAUSE TestAccServiceEndpointAws_oidc
=== RUN   TestAccServiceEndpointAws_requiresImportErrorStep
=== PAUSE TestAccServiceEndpointAws_requiresImportErrorStep
=== CONT  TestAccServiceEndpointAws_basic
=== CONT  TestAccServiceEndpointAws_oidc
=== CONT  TestAccServiceEndpointAws_update
=== CONT  TestAccServiceEndpointAws_requiresImportErrorStep
=== CONT  TestAccServiceEndpointAws_complete
--- PASS: TestAccServiceEndpointAws_basic (167.67s)
--- PASS: TestAccServiceEndpointAws_requiresImportErrorStep (205.00s)
--- PASS: TestAccServiceEndpointAws_update (228.87s)
--- PASS: TestAccServiceEndpointAws_complete (700.02s)
--- PASS: TestAccServiceEndpointAws_oidc (708.76s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        715.875s