Ensure assets get recompiled if cached-assets-hash.txt is empty

What does this MR do and why?

This backports !206131 (merged) to 18-4-stable-ee.

We noticed the canary deployment had stale assets. It appears the downloaded cached-assets-hash.txt was 0 bytes (https://dev.gitlab.org/gitlab/gitlab-ee/-/jobs/30827988#L172). This appeared to have been empty because the cache:assets-hash job failed to upload the build.env artifacts due to rate limiting errors (https://dev.gitlab.org/gitlab/gitlab-ee/-/jobs/30827983), and that job is allowed to fail.

Relates to gitlab-com/gl-infra/production#20599 (closed)

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