Ensure that VariantBuilderMultiSampleVector works with 0 samples / 0 values per sample

-Make sure we don't crash when creating empty VariantBuilderMultiSampleVectors

-Tests for VariantBuilder already exist confirming that the builder will clear
 the relevant field when given an empty VariantBuilderMultiSampleVector
Merge pull request #394 from broadinstitute/dr_test_multi_sample_vector_with_max_size_zero

Ensure that VariantBuilderMultiSampleVector works with 0 samples / 0 values per sample