NetworkManager doesn't clean up resolv.conf file when deactivating device - at least when named-manager has only one configuration in its config list.

Reproducible: always

Steps to reproduce:
1. Open NetworkManager and connect to your favorite wireless network (I guess wired networks are affected, too, but haven't tested it).
2. Deactivate your wireless device - for instance, shut down NetworkManager daemon.

Expected results:
- The corresponding entries (nameserver/search) are removed from /etc/resolv.conf (comments in the code and common sense suggest that this should be the case).

Actual results:
- Nothing happens - the entries corresponding to the deactivated device are still present in /etc/resolv.conf

The bug is in the src/named-manager/nm-named-manager.c:rewrite_resolv_conf() function which silently returns TRUE when passed NULL as a configuration (it assumes that "there is nothing to do" in this case). This is wrong behaviour: instead, rewrite_resolv_conf() should write an empty /etc/resolv.conf in this case (as suggested by nm_named_manager_remove_ip4_config() code). The supplied patch solves the problem.