wrong endtime when first timestamp is null

[Hagellach37]

The /stats/interval endpoints returns a wrong endtime, when there are not contributions for the first time interval bin.

query:

• https://stats.now.ohsome.org/api/stats/adt/interval?startdate=2023-01-01T19:05:00.000Z&enddate=2023-02-01T19:05:00.000Z&interval=P1D

response values for first item:

{
    "0": {
        "changesets": 0,
        "users": 0,
        "roads": 0,
        "buildings": 0,
        "edits": 0,
        "startDate": "2023-01-01T00:00",
        "endDate": "1970-01-03T00:00"
    }
}

endDate should be 2023-01-02T00:00