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This will verify a leak , but is not recommended if unit has no refrigerant left.......
If you find there is no refrigerant, you already know you have a leak, so putting it into a vac will only cause the outside air/humidity to be sucked into the system via that same leak point..... not good.
When one has a leak , the system finally reaches zero standing pressure, but does no go into a vac .
However , the mentioned vac holding test is a technique for testing AFTER the leak has been found /repaired , before recharge to verify repair. This save the hazzle of recharging only to find the leak repair has not been successful................