The current implementation of v:format.json.encode converts the value false to {}, when used on top level. Although the JSON spec allows any value (object, array, true, false, null, numbers) to be on top level. (https://stackoverflow.com/questions/17220924/does-the-top-level-value-in-json-have-to-be-an-array-or-object)

{v:format.json.encode(value: falseVariable)} --> returns {}
{v:format.json.encode(value: trueVariable)} --> returns true

The problem is the following code because empty(false) evaluates to true:

If you think this could safely be removed, I could prepare a pull request.