DRILL-6962: Function coalesce returns an Error when none of the columns in coalesce exist in a parquet file
- Updated UntypedNullVector to hold value count when vector is allocated and transfered to another one;
- Updated RecordBatchLoader and DrillCursor to handle case when only UntypedNull values are present in RecordBatch (special case when data buffer is null but actual values are present);
- Added functions to cast UntypedNull value to other types for use in UDFs;
- Moved UntypedReader, UntypedHolderReaderImpl and UntypedReaderImpl from org.apache.drill.exec.vector.complex.impl to org.apache.drill.exec.vector package.
closes #1614
