If you display the full list, then the empty values are shown in light gray color. You can configure this in Tools - Options - Colors - Font color for attributes without values. Here is an example with all attributes shown, including the empty ones.
You can toggle between these two modes by using the menu option View - Attribute Output Settings - Show only attributes that have values. Or you activate and deactivate the Show Only Attributes That Have Values button .
Although the full list may look overloaded and harder to handle, it might be easier to deal with if you have to add some attributes to an object: Just double-click it, or press F2 or ENTER to edit its value:
In the mode where you see only the attributes that have values, you have to open the Create new attribute dialog for this.
Please remember that the attribute display could also be filtered by the attribute filter configuration. The attribute filter specifies generally what attributes LEX shows in the list, with the Show Only Attributes That Have Values button you configure in addition to that if the empty values are visible or not. So both settings are mixed together.Just jump to the attributes name and double click it or press ENTER to edit the value - after that the attribute is added to the object.