`azuredevops_serviceendpoint_dockerregistry` Enhance nil check

[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: #1144 Enhance nil check,

=== RUN   TestAccServiceEndpointDockerRegistry_CreateAndUpdate
=== PAUSE TestAccServiceEndpointDockerRegistry_CreateAndUpdate
=== CONT  TestAccServiceEndpointDockerRegistry_CreateAndUpdate
--- PASS: TestAccServiceEndpointDockerRegistry_CreateAndUpdate (125.43s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        132.820s

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.)

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