No, the output of the “Aggregate to JSON” module is of variable type string (text), NOT an array!
You should not use the “Length” function to check the length of the resulting string, because it will always be longer than 0. This is because when there are no items, the output of the JSON module string is [] – which has 2 characters in it [ and ]
The preferred way to stop executing the rest of the scenario when the aggregator aggregates no items, is to check the “Stop processing after an empty aggregation” field.