Can you have a zero quantity vf:EconomicResource?
Depends on what you mean by "have", which I think depends on what you want to do with it.
If you want to use the resource in some way, no.
If you want to refer to a resource that once existed, and want to see its history, yes.
We have discussed how to identify resources in https://github.com/valueflows/resource/issues/13 and https://github.com/valueflows/resource/issues/23
Regardless of which form of identification we use, a vf:EconomicResource will have an IRI if and when some vf:Agent chooses to define it. So it will exist, at least in that way, regardless of its quantity (which could initially be zero, and may be zero again off and on, and it can still exist as an IRI which identifies a vf:EconomicResource that existed in tangible form now and then. And you might be interested the history of its appearances and disappearances from tangible form.
If a vf:EconomicResource is identified indirectly, for example by vf:EconomicResourceType and location, with a quantity of 100, and then all 100 are consumed, the identification will need to remain, at least for historical reports. If tomorrow 150 of the vf:EconomicResource fitting that indirect identification appears, are they the same vf:EconomicResource? According to their identifier, they are.
If the vf:EconomicResource was identified by lot id and location, then when 100 are consumed, and 150 more of the same vf:EconomicResourceType but a different lot id appear in the same location, those will not be the same vf:EconomicResource.
If the vf:EconomicResource was identified by a serial number and location, then its quantity will be 1, and when it is gone, whatever resources show up in the same location tomorrow will be different vf:EconomicResources.
These conclusions may seem paradoxical if you are thinking of a resource as a physical object that you can pick up and move to someplace else. But all we got are electronic representations, and that (above) is all they can do. It's another difference between the map and the territory.