Fix error thrown with missing note fragment in DOM

What does this MR do?

• Fix error thrown when loading the diff tab that doesn't have the note associated with the fragment hash in the URL.

Are there points in the code the reviewer needs to double check?

Reproduction:

1. Visit a MR with a note hash that doesn't exist, /namespace/project/merge_requests/x/diffs#note_1234
2. Notice the error throw when the diff tab loads

Why was this MR needed?

• An error is thrown when loading the diff tab that doesn't have the note associated with the fragment hash in the URL.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32888 (closed)

changed the description

marked the checklist item All builds are passing as completed

assigned to @ClemMakesApps

assigned to @MadLittleMods

resolved all discussions

added 1 commit

• 1a896f71 - Fix error thrown with missing note fragment in DOM

Compare with previous version

assigned to @ClemMakesApps

resolved all discussions

resolved all discussions

added 1 commit

• 07a3a69c - Fix error thrown with missing note fragment in DOM

Compare with previous version

resolved all discussions

approved this merge request

merged

mentioned in commit a59165e7

mentioned in issue #33450 (closed)

mentioned in merge request !12011 (merged)

@MadLittleMods: This MR doesn't apply cleanly onto 9-2-stable. Could you please fix the conflicts in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12011?

mentioned in commit 0d554df6