DRILL-6588: Make Sys tables of nullable datatypes
This is to address the generic problem of NULL values being projected as a string because of all datatypes being non-nullable.
This patch only applies to tables backed by the PojoReader (in our case, System tables). Added NonNullable annotations wherever application in any of the System tables, along with a unit test that verifies both nullable and non-nullable datatypes exist in the system tables.
closes #1371
