Guard against nil bindings in solutions.

Author: gkellogg

lib/rdf/query/solutions.rb

@@ -109,7 +109,7 @@ def bindings
     # Duplicates each solution.
     # @return [RDF::Query::Solutions]
     def dup
-      RDF::Query::Solutions.new(self.map(&:dup))
+      RDF::Query::Solutions.new(self.compact.map(&:dup))
     end
 
     ##