Fix non-empty RDF.Graph when adding empty RDF.Description (#8)

lib/rdf/graph.ex

@@ -177,8 +177,13 @@ defmodule RDF.Graph do
     end)
   end
 
-  def add(%__MODULE__{} = graph, %Description{subject: subject} = description),
-    do: do_add(graph, subject, description)
+  def add(%__MODULE__{} = graph, %Description{subject: subject} = description) do
+    if Description.count(description) > 0 do
+      do_add(graph, subject, description)
+    else
+      graph
+    end
+  end
 
   def add(graph, %__MODULE__{descriptions: descriptions, prefixes: prefixes}) do
     graph =

test/unit/graph_test.exs

@@ -98,6 +98,11 @@ defmodule RDF.GraphTest do
       assert graph_includes_statement?(g, {EX.Subject, EX.predicate(), EX.Object})
     end
 
+    test "creating an unnamed graph with an empty description" do
+      g = Graph.new(Description.new(EX.Subject))
+      assert empty_graph?(g)
+    end
+
     test "creating a named graph from another graph" do
       g =
         Graph.new(Graph.new({EX.Subject, EX.predicate(), EX.Object}),