Currently through the GUI you can only configure Project User Input as strings. If you edit the json you can convert it from EsriFieldTypeString to EsriFieldTypeInteger and this works ok as long as you actually capture the project user input. If you leave it blank the app tries to submit an empty string "" rather than a null value. This causes the record submission to fail.
I would like it if the project user input could be all possible values including int/double/string/date etc and if it is not filled in then it should submit null rather than "".