Fix column value populated without a file when using filename override

Fixes #2284

lib/carrierwave/uploader/proxy.rb

@@ -30,7 +30,7 @@ def current_path
       # [String] uniquely identifies a file
       #
       def identifier
-        @identifier || storage.try(:identifier)
+        @identifier || (file && storage.try(:identifier))
       end
 
       ##

spec/uploader/proxy_spec.rb

@@ -29,6 +29,32 @@
     end
   end
 
+  describe '#identifier' do
+    subject { uploader.identifier }
+
+    context "when nothing has been done" do
+      it { is_expected.to be_nil }
+    end
+
+    context "when a file is stored" do
+      before { uploader.store!(test_file) }
+
+      it { is_expected.to eq 'test.jpg' }
+    end
+
+    context "when #filename is set to always return a value" do
+      before do
+        uploader.class_eval do
+          def filename
+            'dummy'
+          end
+        end
+      end
+
+      it { is_expected.to be_nil }
+    end
+  end
+
   describe '#read' do
     subject { uploader.read }