fixed category_by_name() bug

theme/templates/macros/getters.html

 {%- macro category_by_name(catname, ignore_native = False) -%}
 	{%- if ignore_native -%}
-		{{- caller(null, sc.categories[catname], null) -}}
+		{{- caller(None, sc.categories[catname], None) -}}
 	{%- else -%}
 		{%- for nativecat, catarticles in categories -%}
 			{%- if nativecat.name == catname -%}