Do not keep empty strings as crop values in essence picture url options

Having empty strings as EssencePicture crop value (`crop_from` or `crop_size`) causes imagemagick to not crop the image but resize only.

Fixes #1279

app/models/alchemy/essence_picture.rb

@@ -74,8 +74,8 @@ def picture_url_options
 
       {
         format: picture.default_render_format,
-        crop_from: crop_from,
-        crop_size: crop_size
+        crop_from: crop_from.presence,
+        crop_size: crop_size.presence
       }.with_indifferent_access
     end
 

spec/models/alchemy/essence_picture_spec.rb

@@ -126,6 +126,19 @@ module Alchemy
         end
       end
 
+      # Regression spec for issue #1279
+      context 'with crop sizes being empty strings' do
+        before do
+          expect(essence).to receive(:crop_size) { '' }
+          expect(essence).to receive(:crop_from) { '' }
+        end
+
+        it "does not include these crop sizes.", :aggregate_failures do
+          expect(picture_url_options[:crop_from]).to be_nil
+          expect(picture_url_options[:crop_size]).to be_nil
+        end
+      end
+
       context 'without picture assigned' do
         let(:picture) { nil }