Fix Jinja2 filter plugins initialization

_get_filter_plugins() checks FILTER_PLUGINS against None hence
the latter should be initialized to None.

lib/ansible/utils/template.py

@@ -31,7 +31,7 @@ import pwd
 
 # TODO: refactor this file
 
-FILTER_PLUGINS = {}
+FILTER_PLUGINS = None
 _LISTRE = re.compile(r"(\w+)\[(\d+)\]")
 JINJA2_OVERRIDE='#jinja2:'