glib-aux: fix nicks for zero flag in nm_utils_enum_to_str()
nm_utils_enum_to_str() can print flags, that is, combinations of powers of two integers. It also supports nicks, for certain flags. When we have a nick for value zero, then that requires special handling. Otherwise, that zero nick will always show up in the string representation, although, it should only be used if the enum value is exactly zero.