Fix check for ios simulator

When we check if there is running simulator we use ps cax but if the simulator is killed for a short period of time after that there is a process listed when using ps cax and we decide that there is a running simulator and then we try to use it but ios-sim-portable returns null. We should check if the simulator returned from ios-sim-portable is not null or undefined in order to continue to work with it.

Author: TsvetanMilanov