Support setting false-y values for cache_client

[danielwestendorf]

Given an app that

• Does not use this gem's disabled? functionality
• Rails.cache is a file store

This gem then prepends every job execution with a file read. This isn't a big deal, but setting ActiveJob::TrafficControl.cache_client = false has no effect, despite the library having support for a false-y cache_client. This is due to usage of ||= and the default of Rails.cache.