backends: Don't try to process userAccountControl unless configured.
backends: Don't try to process userAccountControl unless configured.

This fixes an exception in manage.py sync_ldap_user_data if
userAccountControl is not setup on the system yet.

shubhamdhama/zulip,rishig/zulip,tommyip/zulip,brainwane/zulip,zulip/zulip,hackerkid/zulip,tommyip/zulip,brainwane/zulip,hackerkid/zulip,showell/zulip,showell/zulip,rishig/zulip,timabbott/zulip,zulip/zulip,eeshangarg/zulip,dhcrzf/zulip,showell/zulip,eeshangarg/zulip,tommyip/zulip,zulip/zulip,timabbott/zulip,eeshangarg/zulip,tommyip/zulip,dhcrzf/zulip,timabbott/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,dhcrzf/zulip,synicalsyntax/zulip,rishig/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,hackerkid/zulip,hackerkid/zulip,rishig/zulip,tommyip/zulip,synicalsyntax/zulip,eeshangarg/zulip,timabbott/zulip,dhcrzf/zulip,punchagan/zulip,rht/zulip,shubhamdhama/zulip,shubhamdhama/zulip,timabbott/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,punchagan/zulip,dhcrzf/zulip,kou/zulip,andersk/zulip,rht/zulip,dhcrzf/zulip,brainwane/zulip,synicalsyntax/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,showell/zulip,zulip/zulip,rishig/zulip,punchagan/zulip,synicalsyntax/zulip,shubhamdhama/zulip,rht/zulip,tommyip/zulip,showell/zulip,shubhamdhama/zulip,kou/zulip,kou/zulip,tommyip/zulip,synicalsyntax/zulip,brainwane/zulip,punchagan/zulip,punchagan/zulip,rishig/zulip,timabbott/zulip,showell/zulip,andersk/zulip,shubhamdhama/zulip,shubhamdhama/zulip,brainwane/zulip,hackerkid/zulip,showell/zulip,kou/zulip,kou/zulip,brainwane/zulip,dhcrzf/zulip,punchagan/zulip,zulip/zulip,timabbott/zulip,synicalsyntax/zulip,zulip/zulip,kou/zulip,rishig/zulip,eeshangarg/zulip,andersk/zulip