allow nil template source

lib/liquid/template.rb

@@ -108,7 +108,7 @@ module Liquid
     def parse(source, options = {})
       parse_context = configure_options(options)
 
-      unless source.valid_encoding?
+      if source.is_a?(String) && !source.valid_encoding?
         raise SyntaxError, parse_context.locale.t("errors.syntax.invalid_template_encoding")
       end
 

test/integration/template_test.rb

@@ -349,4 +349,9 @@ class TemplateTest < Minitest::Test
 
     assert_equal('Liquid syntax error: Invalid template encoding', e.message)
   end
+
+  def test_allows_nil_as_source
+    template = Template.parse(nil)
+    assert_equal('', template.render)
+  end
 end