Guard against nil as value

What does this MR do and why?

#fetch tests for key existence, so if the value of options_retry[:when] is, for some reason, set as nil, we will see NoMethodErrors elsewhere in the app, when the intent is clearly to always return the default value of ['always']

Spotting this while bopping around Sentry: https://new-sentry.gitlab.net/organizations/gitlab/issues/1536/

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