docs: `lastMeasurementAt` is actually optional but docs state it's not

[hf-kklein]

Expected Behavior

The docs should represent the data to be expected from the API.

Current Behavior

There are boxes which have no measurements.
E.g. boxId: 5c68119ca100840019d8d3c8
https://api.opensensemap.org/boxes/5c68119ca100840019d8d3c8

Those boxes have no lastMeasurementAt.
The docs say, this property is not optional:
https://docs.opensensemap.org/#api-Boxes-getBox

But it is optional. Not all boxes have the property set

Possible Solution

• Mark as optional in the docs
• set default value server side (e.g. null)

Steps to Reproduce (for bugs)

Try to bind https://api.opensensemap.org/boxes/5c68119ca100840019d8d3c8 to a model which has lastMeasurementAt as not nullable required field.