gui: Fix Glib-GObject-WARNING when closing the active dataset.

gui: Fix Glib-GObject-WARNING when closing the active dataset.

The DATASET CLOSE command, when it acts on the active dataset, just
removes the active dataset's name, changing it to the empty string.
(This is the documented behavior.)  However, the GUI relies on
every dataset having a name, so this caused the following warning:

GLib-GObject-WARNING **: value """" of type `gchararray' is invalid
or out of range for property `id' of type `gchararray'

This commit fixes the problem by giving any unnamed dataset a name
after running syntax.