Fix link anchors for synthetic system notes

What does this MR do and why?

System notes from label, milestone, and state changes are represented by synthetic system notes that have an alphanumeric ID. We should not call getIdFromGraphQLId on these because these would return null.

This is a regression from 3255cf6d

MR acceptance checklist

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Screenshots or screen recordings

Before:

After:

How to set up and validate locally

1. Change some labels on an issue
2. Hover over the timestamp link and check the anchor
3. You can also click on the timestamp and it should get highlighted and the browser URL updated to the correct link with anchor