Add CarrierWave::Storage::Fog::File#empty?, for consistency with SanitizedFile

Refs. #1926 (comment)

Author: mshibuya

lib/carrierwave/storage/fog.rb

@@ -309,6 +309,15 @@ def size
           file.nil? ? 0 : file.content_length
         end
 
+        ##
+        # === Returns
+        #
+        # [Boolean] whether the file is non-existent or empty
+        #
+        def empty?
+          !exists? || size.zero?
+        end
+
         ##
         # Check if the file exists on the remote service
         #

spec/storage/fog_helper.rb

@@ -704,6 +704,32 @@ def check_file
           @fog_file.copy_to('uploads/new_path.jpg')
         end
       end
+
+      describe '#empty?' do
+        it "returns false when the file exists" do
+          expect(@fog_file.empty?).to be false
+        end
+
+        context "when the remote file doesn't exist" do
+          before do
+            @fog_file.delete
+          end
+
+          it "returns true" do
+            expect(@fog_file.empty?).to be true
+          end
+        end
+
+        context "when the remote file has no content" do
+          before do
+            @fog_file = @storage.store!(CarrierWave::SanitizedFile.new(StringIO.new('')))
+          end
+
+          it "returns true" do
+            expect(@fog_file.empty?).to be true
+          end
+        end
+      end
     end
   end
 end