Attachment default_url should allow nil

lib/paperclip/interpolations.rb

@@ -30,6 +30,7 @@ def self.all
     # You can pass a method name on your record as a symbol, which should turn
     # an interpolation pattern for Paperclip to use.
     def self.interpolate(pattern, *args)
+      return if pattern.nil?
       pattern = args.first.instance.send(pattern) if pattern.is_a? Symbol
       result = pattern.dup
       interpolators_cache.each do |method, token|

lib/paperclip/url_generator.rb

@@ -18,6 +18,7 @@ def for(style_name, options)
       interpolated = attachment_options[:interpolator].interpolate(
         most_appropriate_url, @attachment, style_name
       )
+      return unless interpolated
 
       escaped = escape_url_as_needed(interpolated, options)
       timestamp_as_needed(escaped, options)

spec/paperclip/attachment_spec.rb

@@ -278,6 +278,21 @@
       end
     end
 
+    context 'without an Attachment and default_url nil' do
+      before do
+        rebuild_model default_url: nil
+        @dummy = Dummy.new
+      end
+
+      it 'returns false when asked exists?' do
+        assert [email protected]?
+      end
+
+      it '#url returns the default_url' do
+        expect(@dummy.avatar.url).to eq nil
+      end
+    end
+
     context "on an Attachment" do
       before do
         @dummy = Dummy.new