Fix an issue with blank visibility

Followup to ManageIQ#23803
Fixes an issue seen when testing UI usage in ManageIQ/manageiq-ui-classic#9954

The client of this code shouldn't need to check if the visibility is missing
the roles/groups key.

Author: jrafanie

app/models/miq_widget.rb

@@ -456,6 +456,7 @@ def visibility_values
 
     typ = visibility.keys.first
     values = visibility.values.flatten
+    return [] if typ.blank? || values.blank?
 
     # Convert ids to useful strings for display
     case typ

spec/models/miq_widget_spec.rb

@@ -961,6 +961,11 @@ def add_dashboard_for_user(db_name, userid, group)
       widget = FactoryBot.build(:miq_widget, :visibility => {:roles => ['_ALL_']})
       expect(widget.visibility_values).to eq([])
     end
+
+    it "handles blank visibility" do
+      widget = FactoryBot.build(:miq_widget, :visibility => {})
+      expect(widget.visibility_values).to eq([])
+    end
   end
 end