Fix the initial weather request in case last position is null

calculateGreatCircleDistance returns false immediately if any of the
arguments was null.

ref T324

src/blackgui/components/weathercomponent.cpp

@@ -174,7 +174,7 @@ namespace BlackGui
 
             if (CWeatherScenario::isRealWeatherScenario(scenario))
             {
-                if (!useOwnAcftPosition ||
+                if (m_lastOwnAircraftPosition.isNull() ||
                         calculateGreatCircleDistance(position, m_lastOwnAircraftPosition).value(CLengthUnit::km()) > 20)
                 {
                     requestWeatherGrid(position);