DRILL-665: Handle null values in case expressions (contd).

1. Added functions for converting REQUIRED holder into NULLABLE holder where the minorType is same.
2. Update in Optiq->Drill literal conversion. First check if it null type, before parsing the literal value. Parsing literal value will cause NPE if the type is NULL.
3. Changed getReturnType of IfExpression to consider the nullable types of THEN and ELSE expressions.
4. Added testcases.