[KoushikSagarr]

What github issue is this PR for, if any?

Resolves #7265

What changed, and why?

• Replaced "/dev/null" with File::NULL in the Bugsnag logger initializer to resolve the Style/FileNull offense.
• Removed the corresponding resolved offense from .standard_todo.yml.
• Kept changes strictly limited to the Bugsnag initializer and its StandardRB baseline entry.

This is another small, independently reviewable contribution toward completing the project's StandardRB compliance work.

How is this tested? (please write rspec and jest tests!) 💖💪

• StandardRB: 0 offenses.
• RSpec: spec/services/backfill_followupable_service_spec.rb — 2 examples, 0 failures.
• Rails boot verification: bundle exec rails runner "p Bugsnag.configuration.logger" passed and confirmed Logger instantiation.
• Git diff: git diff --check passed with no whitespace errors.
• Jest: Not applicable — this change is limited to a Ruby initializer and has no frontend JavaScript behavior.
• No dedicated initializer spec exists; the Rails runner boot test verifies the logger configuration.

Screenshots please :)

Not applicable — this is a backend initializer and configuration cleanup.

Feelings gif (optional)

One more clean contribution, one more step forward. StandardRB backlog conquered! 💪