Added check for available drivers in interface function (for CP)

This should prevent the CP driven combine to stand still even though there is no active AD driver ready to come and unload.
This is what keeps blocking the combine on manual unload mode.

FS19_AutoDrive/scripts/AutoDriveCombineMode.lua

@@ -85,7 +85,7 @@ function AutoDrive:callDriverToCombine(combine)
 end;
 
 function AutoDrive:combineIsCallingDriver(combine)
-    return (combine.ad ~= nil) and (combine.ad.tryingToCallDriver or combine.ad.driverOnTheWay);
+    return (combine.ad ~= nil) and ((combine.ad.tryingToCallDriver and ADTableLength(AutoDrive.waitingUnloadDrivers) > 0) or combine.ad.driverOnTheWay);
 end;
 
 function AutoDrive:handleReachedWayPointCombine(vehicle)