Fix handling Service Ping response with only usage_data_id

What does this MR do and why?

Related to #337497 (closed)

https://gitlab.com/gitlab-services/version-gitlab-com/-/merge_requests/630 fixed Version App not sending usage_data_id when there's not enough payloads from a given GitLab instance to send back full DevOps metrics in a conv_index Hash. This only happens for a very first Service Ping from a new instance.

This turned out to not be enough to fix #337497 (closed) and ServicePing::SubmitService now tries to save an empty DevOpsReport::Metric record when only usage_data_id is present in a response.

This MR fixes this behavior by omitting usage_data_id in a check if metrics are present in a response.

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How to set up and validate locally

1. Reset GDK data with gdk reset-data
2. Set ServicePing::SubmitService.base_url to point to your local Version App instance.
3. Send Service Ping to local Version App instance by running ServicePing::SubmitService.new.execute

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