Bad assertion in serializePartitionKey when shortKeys are enabled

[tolbertam]

When running with assertions enabled and no schema is provided via '-c' toJson will fail while serializing a partition key because it expects the number of key components to match the number of partition columns in the metadata (which are empty):

Exception in thread "main" java.lang.AssertionError
    at com.csforge.sstable.JsonTransformer.serializePartitionKey(JsonTransformer.java:83)
    at com.csforge.sstable.JsonTransformer.serializePartition(JsonTransformer.java:149)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
    at java.util.Iterator.forEachRemaining(Iterator.java:116)
    at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at com.csforge.sstable.JsonTransformer.toJson(JsonTransformer.java:58)
    at com.csforge.sstable.SSTable2Json.main(SSTable2Json.java:109)
    at com.csforge.sstable.Driver.main(Driver.java:17)