Use an empty list instead of None for default executors list.
Use an empty list instead of None for default executors list.

Without this, a type exception is thrown when the code tries to
iterate over the non-iterable None default value.

swift-lang/swift-e-lab,Parsl/parsl,Parsl/parsl,Parsl/parsl,Parsl/parsl,swift-lang/swift-e-lab