Improve to_date, to_time and to_timestamp and corresponding cast functions to handle empty string when `drill.exec.functions.cast_empty_string_to_null` option is enabled

When `drill.exec.functions.cast_empty_string_to_null` option is enabled

`to_date`, `to_time` and `to_timestamp` functions while converting string to according type in case if null or empty string values are passed will return NULL (to avoid CASE clauses which are littering a query and will work in accordance with their respective CAST counterparts) for both cases.

CASTs will  be handled in a similar way (uniformly with numeric types):

 CAST empty string to null (in case of enabled option) will be supported by DATE, TIME, TIMESTAMP, INTERVAL YEAR, INTERVAL MONTH and INTERVAL DAY functions in addition to numeric types.

For documentation

TBA