Compact `files_to_load` before checking if it's empty

My guard file is setup to run `acceptance/<file>_spec.rb`,
`controllers/<file>_rspec.rb` and `routing/<file>_routing_spec.rb`, when
`<file>` is saved.

Some of my controllers have routing specs, some doesn't. In case they
doesn't spin will raise an error, because it can't invoke `empty?` on
nil.

This patch compmacts the `files_to_load` list, before testing if it's
empty. This fixes the problem described above.