Last seen timestamp validator fix

[grafnu]

No description provided.

[noursaidi]

Many devices are having this unexpected error:

  "errors" : [ {
    "message" : "java.lang.IllegalArgumentException",
    "detail" : "null_pointset: java.lang.IllegalArgumentException @Validator.validateUpdate(Validator.java:484)",
    "category" : "validation.device.schema",
    "timestamp" : "2022-12-02T13:53:36Z",
    "level" : 500
  } ]

[grafnu]

Ok -- another case of a subType default... should be fixed now. I ran it on IN-BLR-BCPEK and all seemed well. PTAL.