nnimap.el: Handle nil arg to nnimap-request-group

nnimap.el:  Handle nil arg to nnimap-request-group

* nnimap.el (nnimap-request-group):  Handle nil "info" arg.  This arg
isn't always passed in, check it's not nil before making it into a list.
The active arg will also be nil if the group is new, check for that.