DRILL-7436: Fix record count, vector structure issues in several operatorsAdds additional vector checks to the BatchValidator.Enables checking for the following operators:* FilterRecordBatch* PartitionLimitRecordBatch* UnnestRecordBatch* HashAggBatch* RemovingRecordBatchFixes vector count issues for each of these.Fixes empty-batch (record count = 0) handling in several of theabove operators. Added a method to VectorContainer to correctlycreate an empty batch. (An empty batch, counter-intuitively,needs vectors allocated to hold the 0 value in the firstposition of each offset vector.)Disables verbose logging for MongoDB tests. Details are written tothe log rather than the console.Disables two invalid Mongo tests. See DRILL-7428.Adjusts the expression tree materializer to not add the LATE typeto Union vectors. (See DRILL-7435.)Ensures that Union vectors contain valid vectors for each subtype.The present fix is a work-around, see DRILL-7434 for a betterlong-term fix.Cleans up code formatting and other minor issues in each file touchedduring the fixes in this PR.