Fixing #697 with better exception

When including a template which is not defined, the exception raised is
*undefined method `split` for nil:NilClass*

This occurs for a scenario like the following:
`{% include nil %}`
or
`{% include undefined-var %}`

Making the code raise an argument error to allow better understanding of
the include error

test/integration/tags/include_tag_test.rb

@@ -217,6 +217,17 @@ class IncludeTagTest < Minitest::Test
     end
   end
 
+  def test_render_raise_argument_error_when_template_is_undefined
+    assert_raises(Liquid::ArgumentError) do
+      template = Liquid::Template.parse('{% include undefined_variable %}')
+      template.render!
+    end
+    assert_raises(Liquid::ArgumentError) do
+      template = Liquid::Template.parse('{% include nil %}')
+      template.render!
+    end
+  end
+
   def test_including_via_variable_value
     assert_template_result "from TestFileSystem", "{% assign page = 'pick_a_source' %}{% include page %}"