Only clear cache when view paths are specified
Only clear cache when view paths are specified

Currently, `clear_cache_if_necessary` is executed even if view paths are
not set like `rails console`.
If the watcher class is `EventedFileUpdateChecker` and the watch
directories are empty, the application root directory will watch. This
is because listen uses the current directory as the default watch directory.
https://github.com/guard/listen/blob/8d85b4cd5788592799adea61af14a29bf2895d87/lib/listen/adapter/config.rb#L13

As a result, `node_modules` also watch. This cause a warning of `listen`.
Ref: https://github.com/rails/rails/pull/36377#issuecomment-498399576

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