azuredevops_serviceendpoint_dockerregistry Enhance nil check

[xuzhang3]

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• 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.
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• 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
• No

Does this introduce a breaking change?

• Yes
• No

Any relevant logs, error output, etc?

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