Allow an empty or null base URI

app/concerns/liquid_interpolatable.rb

@@ -129,10 +129,11 @@ module LiquidInterpolatable
     # userinfo, host, port, registry, path, opaque, query, and
     # fragment.
     def to_uri(uri, base_uri = nil)
-      if base_uri
-        Utils.normalize_uri(base_uri) + Utils.normalize_uri(uri.to_s)
-      else
+      case base_uri
+      when nil, ''
         Utils.normalize_uri(uri.to_s)
+      else
+        Utils.normalize_uri(base_uri) + Utils.normalize_uri(uri.to_s)
       end
     rescue URI::Error
       nil

spec/concerns/liquid_interpolatable_spec.rb

@@ -119,6 +119,15 @@ describe LiquidInterpolatable::Filters do
       @agent.interpolation_context['s'] = 'foo/index.html'
       expect(@agent.interpolated['foo']).to eq('/dir/foo/index.html')
     end
+
+    it 'should normalize a URI value if an empty base URI is given' do
+      @agent.options['foo'] = '{{ u | to_uri: b }}'
+      @agent.interpolation_context['u'] = "\u{3042}"
+      @agent.interpolation_context['b'] = ""
+      expect(@agent.interpolated['foo']).to eq('%E3%81%82')
+      @agent.interpolation_context['b'] = nil
+      expect(@agent.interpolated['foo']).to eq('%E3%81%82')
+    end
   end
 
   describe 'uri_expand' do