Skip trace stream loading when coverage regex is absent

What does this MR do and why?

Ci::Build#update_coverage now returns early when coverage_regex is blank instead of calling trace.extract_coverage. This avoids unnecessary work since most builds have no coverage keyword in their CI config.

Before this change, extract_coverage was called unconditionally. Although it returns early when the regex is blank, it first builds the trace stream: loading the job_artifacts_trace association and either opening an archived trace file or constructing a ChunkedIO with an extra query. With the guard clause, none of this overhead runs for builds without a coverage regex, keeping the behavior identical while skipping work.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/605350

Screenshots or screen recordings

Not applicable, backend-only change.

How to set up and validate locally

1. In a Rails console, pick a build without a coverage regex and turn on SQL logging:

   build = Ci::Build.where(coverage_regex: nil).last
   ActiveRecord::Base.logger = Logger.new($stdout)

2. Run the method:

   build.update_coverage # => nil

   Before this change the log shows queries against ci_job_artifacts and ci_build_trace_chunks. After it, no queries run.

3. Coverage stays nil, as before, since there was no regex to extract with:

   build.coverage # => nil

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