users, fingerprint: Avoid accidental NULL css class name
Since we don't define a style for the ENROLL_STATE_NORMAL state, we were using "" as a placeholder in the ENROLL_STATE_CLASSES list, which is an empty string for gtk_widget_{add/remove}_css_class.
Let's use "normal" instead and comment it as "undefined", since we don't define any css styling for it.
Fixes the following criticals during fingerprint enrollment.
15:25:20.6575 Gtk[307443]:CRITICAL: gtk_widget_is_ancestor: assertion 'GTK_IS_WIDGET (widget)' failed
15:25:21.6477 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:21.6477 Gtk[307443]:CRITICAL: gtk_widget_add_css_class: assertion 'css_class[0] != '\0'' failed
15:25:23.3037 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:24.0548 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:24.0549 Gtk[307443]:CRITICAL: gtk_widget_add_css_class: assertion 'css_class[0] != '\0'' failed
15:25:25.0959 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:25.4640 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:25.7319 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:26.1519 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:26.6959 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:27.3641 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:27.7800 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:28.5305 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed
15:25:28.5306 Gtk[307443]:CRITICAL: gtk_widget_add_css_class: assertion 'css_class[0] != '\0'' failed
15:25:29.7042 Gtk[307443]:CRITICAL: gtk_widget_remove_css_class: assertion 'css_class[0] != '\0'' failed