load header extra file if request comes from micro site

use blank header_extra template instead of if statement

lms/templates/header_extra.html

@@ -0,0 +1,3 @@
+<%
+# This template is left blank on purpose. It can be overridden by microsites.
+%>

lms/templates/main.html

@@ -94,7 +94,7 @@ from branding import api as branding_api
     style_overrides_file = None
 
   else:
-    header_extra_file = None
+    header_extra_file = microsite.get_template_path('header_extra.html')
 
     if settings.FEATURES['IS_EDX_DOMAIN'] and not is_microsite():
         header_file = microsite.get_template_path('navigation-edx.html')