Fix empty fuel pump bug in GasStationRefueling sample (#44)

Adds check for empty fuel pump before attempting to draw fuel
in GasStationRefueling sample, preventing an invalid get request
for 0 fuel. Note: this bug only materialises when attributes are
changed from current values such that an empty fuel pump occurs
during simulation.

Author: sean-reed

src/Samples/GasStationRefueling.cs

@@ -55,7 +55,7 @@ private IEnumerable<Event> Car(string name, Simulation env, Resource gasStation,
 
         // Get the required amount of fuel
         var litersRequired = FuelTankSize - fuelTankLevel;
-        if (litersRequired > fuelPump.Level) {
+        if (litersRequired > fuelPump.Level && fuelPump.Level > 0) {
           var level = fuelPump.Level;
           yield return fuelPump.Get(level); // draw it empty
           yield return env.Timeout(TimeSpan.FromSeconds(level / RefuelingSpeed));