GH-343: Take into consideration VerticalAccuracy on iOS Altitude (#344)

* Fix for #343

Only set altitude if it's available (VerticalAccuracy is negative when it isn't according to iOS doc)

* Use: (double?)

Xamarin.Essentials/Types/LocationExtensions.ios.cs

@@ -25,7 +25,7 @@ internal static Location ToLocation(this CLLocation location) =>
             {
                 Latitude = location.Coordinate.Latitude,
                 Longitude = location.Coordinate.Longitude,
-                Altitude = location.Altitude,
+                Altitude = location.VerticalAccuracy < 0 ? (double?)null : location.Altitude,
                 Accuracy = location.HorizontalAccuracy,
                 TimestampUtc = location.Timestamp.ToDateTime()
             };