May show painted by: (null) in building info window

[Ranran(retired)]

Since the building info window was updated, it now shows "(null)" when there is no author information for a building.
PR#600 changes nothing to be displayed in that case.

[Sirius]

Nice, thanks for this change

Might we instead show "unknown author"? This might give a more consistent look. Consistency makes UIs easier to read.
But that's just a minor thing.
Thank you for the contribution in any case

[jamespetts]

Thank you for this and apologies for the delay: I have now incorporated this.