Eliminate type error on search_s

Guard the list comprehension to eliminate 'None' valued keys from
the attrlist from causing an exception when trying to iterate on
the None singleton.

related-bug: #1231772
Change-Id: I823a53d100b5e4210c767507e3233b61ed372068

keystone/common/ldap/core.py

@@ -556,7 +556,8 @@ def search_s(self, dn, scope, query, attrlist=None):
         # allows us to set mapped attributes to "None" as defaults in config.
         # Without this filtering, the ldap query would raise a TypeError since
         # attrlist is expected to be an iterable of strings.
-        attrlist = [attr for attr in attrlist if attr is not None]
+        if attrlist is not None:
+            attrlist = [attr for attr in attrlist if attr is not None]
         LOG.debug(_(
             'LDAP search: dn=%(dn)s, scope=%(scope)s, query=%(query)s, '
             'attrs=%(attrlist)s') % {