Fix nil polymorphic, with_deleted, belongs_to

When association is polymorphic and association is not set, finder
should return nil

lib/acts_as_paranoid/associations.rb

@@ -15,8 +15,10 @@ def belongs_to_with_deleted(target, options = {})
         if with_deleted
           class_eval <<-RUBY, __FILE__, __LINE__
             def #{target}_with_unscoped(*args)
-              return #{target}_without_unscoped(*args) unless association(:#{target}).klass.paranoid?
-              association(:#{target}).klass.with_deleted.scoping { #{target}_without_unscoped(*args) }
+              association = association(:#{target})
+              return nil if association.options[:polymorphic] && association.klass.nil?
+              return #{target}_without_unscoped(*args) unless association.klass.paranoid?
+              association.klass.with_deleted.scoping { #{target}_without_unscoped(*args) }
             end
             alias_method_chain :#{target}, :unscoped
           RUBY

test/test_associations.rb

@@ -58,6 +58,19 @@ def test_belongs_to_polymorphic_with_deleted
     assert_equal paranoid_time, paranoid_has_many_dependant.paranoid_time_polymorphic_with_deleted(true)
   end
 
+  def test_belongs_to_nil_polymorphic_with_deleted
+    paranoid_time = ParanoidTime.first
+    paranoid_has_many_dependant = ParanoidHasManyDependant.create!(:name => 'dependant!', :paranoid_time_polymorphic_with_deleted => nil)
+
+    assert_nil paranoid_has_many_dependant.paranoid_time
+    assert_nil paranoid_has_many_dependant.paranoid_time_polymorphic_with_deleted
+
+    paranoid_time.destroy
+
+    assert_nil paranoid_has_many_dependant.paranoid_time(true)
+    assert_nil paranoid_has_many_dependant.paranoid_time_polymorphic_with_deleted(true)
+  end
+
   def test_only_find_associated_records_when_finding_with_paranoid_deleted
     parent = ParanoidBelongsDependant.create
     child = ParanoidHasManyDependant.create