Imagine an arbitrary list of items. To execute a command, the user has to select one of these items in a combobox (if there are no items, the combobox is not shown at all). Should the combobox be enabled even if the user can't change anything, because the combobox just contains one item?
If there is only one option selectable, pre-fill the combobox, and remove the ability to make changes by greying it out.
It's context sensitive without messing with the user by actively showing them what is going on but making sure they don't think that they can change it, and without hiding things from them/making them look for it.
Not showing the combobox when there are no items is likely to confuse your customers. If your customers come to expect it to be there, you should always have it there - even if there are no options to select. You could always add an option called "No valid option" or something along those lines.
If you do this then when there is one option the answer is simple. Show only that one option.