exception on empty instance group

[coryb]

Exception: n must be positive
java.lang.IllegalArgumentException: n must be positive
at java.util.Random.nextInt(Random.java:265)
at com.netflix.simianarmy.basic.chaos.BasicChaosInstanceSelector.select(BasicChaosInstanceSelector.java:61)
at com.netflix.simianarmy.basic.chaos.BasicChaosMonkey.doMonkeyBusiness(BasicChaosMonkey.java:89)

If the instance group size is 0 then we call Random.nextInt(0) which throws an exception, the argument must be > 0.

We should return null when instance group size is 0.