The minibar is empty.
VUO-to — two syllables, stress on the first. 'uo' is a diphthong.
If the minibar was not stocked before your arrival. Hotels should have it ready for check-in.
'Vuoto' (empty) is the opposite of 'pieno' (full). 'Il minibar è vuoto' = the minibar is empty. This is unusual and suggests either a stocking oversight or a previous guest drank everything and housekeeping has not restocked.
Il minibar non è stato rifornito.
The minibar has not been restocked.
More formal and precise way to report the issue
Mancano le acquette nel minibar.
The small water bottles are missing from the minibar.
Specify what is missing if only some items are absent
Quando viene rifornito il minibar?
When is the minibar restocked?
Ask for the restocking schedule if you plan to use it
Italian hotel minibars are sometimes emptied and deactivated in lower-budget rooms or when the hotel is not at full capacity, to save electricity. If the minibar is important to you, confirm at check-in that it will be stocked.