Prevent error when adding line references if there isn't a matching target line

What does this MR do and why?

For #519154 (closed)

Fixes an issue when Linked Files and a line_code URL fragment are used together that causes an uncaught fatal error when attempting to add line references.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
Screencast_from_2025-02-13_17-09-40	Screencast_from_2025-02-13_17-07-29

How to set up and validate locally

See the linked issue for full replication steps.

Truncated replication steps:

• Visit a diff using both a line_code URL fragment and the Linked File feature (?file=[sha])
  • Your URL should look something like https://gdk.test:3443/ns/project/-/merge_requests/1/diffs?file=[sha]#[same sha]_[line number]_[line_number]