Run image compress even if Rails.application.assets is nil

[iggant]

Since https://github.com/rails/sprockets-rails/commit/d7c7ee19991c565eb77ee143be2d009ba4472122#diff-b295f4f07d2e863f15e53eb2047cfd1f we can't rely on app.assets for register_preprocessor

When compile=false and we precompile assets register_preprocessor?(app) method will always return false