Resolve "Page editor fatals (AssertionError) in `ComputedUrlWithQueryString` when a component's image references a deleted file"

See issue description. Trivial guard added.

AI-Generated: Yes

Testing instructions

• New test fails on CI: 40d965fe → https://git.drupalcode.org/project/canvas/-/pipelines/856932
• Fix makes CI pass: caadda92 → https://git.drupalcode.org/project/canvas/-/pipelines/856979

Closes #3591702 (closed)