Callback parameter for TestEditorService() is optional

[dan-silver]

Since the callback property is assigned an empty function in the constructor of TestEditorService, we shouldn't have to pass one in every time.

[mention-bot]

@dan-silver, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bpasero and @egamma to be potential reviewers.

[msftclas]

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[bpasero]

Thanks, LGTM