Quick option would be to use a set node and for the inventory value use a Ternary operation so it would use the value of inventory if it exists and if not it uses a 0.
{{$json["inventory"] ? $json["inventory"] : 0}}
An example of this in a workflow is below.
Another option would be to use an if node to check if it is empty then use a set node to set the value and merge the outputs together like the example below.