DRILL-4346: Handle NumberFormatException when casting empty string to int in hbase/maprdb
- Made replacing of functions casting for nullable input varbinary and var16char types are working (in CastFunctions.java setting 'drill.exec.functions.cast_empty_string_to_null = true' didn't work);
- Added new generated classes for casting varbinary and var16char to int (CastEmptyStringVarTypesToNullableNumeric template and Casts.tdd data);
- Created a test (in TestHBaseQueries.java) for checking an empty string to integer casting in maprdb/hbase table;
- Small design changes according to comments in review (import statements, prefixes ...).
This closes #384
