Allow for no hits when hit testing.
Allow for no hits when hit testing.

This can happen e.g. if mouse is captured and moves out of bounds of
window. Fixes #265.

jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,SuperJMN/Avalonia,MrDaedra/Avalonia,kekekeks/Perspex,AvaloniaUI/Avalonia,jazzay/Perspex,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Perspex,AvaloniaUI/Avalonia,danwalmsley/Perspex,grokys/Perspex,wieslawsoltes/Perspex,SuperJMN/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,wieslawsoltes/Perspex,susloparovdenis/Avalonia,wieslawsoltes/Perspex,OronDF343/Avalonia,AvaloniaUI/Avalonia,akrisiun/Perspex,AvaloniaUI/Avalonia,SuperJMN/Avalonia,susloparovdenis/Perspex,jkoritzinsky/Avalonia,grokys/Perspex,SuperJMN/Avalonia,susloparovdenis/Avalonia,jkoritzinsky/Avalonia,susloparovdenis/Perspex,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,Perspex/Perspex,Perspex/Perspex,jkoritzinsky/Avalonia,SuperJMN/Avalonia,kekekeks/Perspex,OronDF343/Avalonia,punker76/Perspex,MrDaedra/Avalonia