Consider `None`-equivalent value for missing data points in weather

[sebastian-peter]

When no weather data is present for a coordinate/time combination, specific values (e.g. 0°K for empty temperature) are provided.

https://github.com/ie3-institute/simona/blob/ad824fc592b772236782c9b44d1d79af2b494d14/src/main/scala/edu/ie3/simona/service/weather/WeatherSource.scala#L513-L518

We should have a discussion on whether this is the best way to solve this, in contrast to passing on None (or equivalent) to SPAs.