Add null checks to create*Block

Make the errors more explicit for calls like this, which do not do what the caller expects due to varargs semantics in Java:

  createStringsBlock(null)

Also, remove the @Nullable annotation as it's ambiguous: does it refer to the varargs parameter or to each of the varargs elements.
Upgrade to commons-math 3.2
Poissonized Sampling With Replacement
Change SampleOperator to use sample weight framework
Fix materialization of rows with zero sample weight
Fix warnings in AbstractTestQueries
Add support for rescaled poissonized sampling