FIX: raise exception when getting dimensions of missing image

- follow-up on 0eacd45ab15cbd20ed9f444fd447886a7fc6dccb

spec/components/image_sizer_spec.rb

@@ -24,10 +24,6 @@ describe ImageSizer do
     expect(ImageSizer.resize('100', '101')).to eq([100, 101])
   end
 
-  it 'returns nil if no width or height are provided' do
-    expect(ImageSizer.resize).to eq(nil)
-  end
-
   describe 'when larger than the maximum width' do
 
     before do