Bug Fix - `azuredevops_serviceendpoint_kubernetes` - Fix potential nil exception

[xuzhang3]

All Submissions:

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

What about the current behavior has changed?

Issue Number: #1088

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

• [ ] Yes
• [x] No

Does this introduce a breaking change?

• [ ] Yes
• [x] 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   TestAccEnvironmentKubernetes_CreateAndUpdate
=== PAUSE TestAccEnvironmentKubernetes_CreateAndUpdate
=== CONT  TestAccEnvironmentKubernetes_CreateAndUpdate
--- PASS: TestAccEnvironmentKubernetes_CreateAndUpdate (124.17s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        125.519s