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Tested in SMW 1.7.0.2:

Produces "0" when there are no results:

{{#ask: [[Category:Existing category]] | format=count }} 
{{#ask: [[Existing property name::any property value]] | format=count }}
{{#ask: [[Non-existing property name::any property value]] | format=count }}

Produces nothing when there are no results:

{{#ask: [[Category:Non-existing category]] | format=count }}