When starting a process model from the web api, the process variable is []. Below I have attached the result for your reference.
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For process model variables you need to pass the data in json format in the body if the body of the API is null that means the inputs are empty to the process model . So you will get null values in process variables.
For process model (which has only query database step that too using sql query) it doesn't have any parameter to be passed. It only gives output and that I will be saving in the process variable and i am trying to fetch it in the web api.