* fix(query): fix timediff args cannot be integer values

TD-14798

* fix(query): fix timediff/timetruncate/to_unixtimestamp handling NULL value and empty string

TD-14802
TD-14803

* refactor(query): constant NULL value processing logic

TD-14802
TD-14803

* refactor(query): support timefunction arithmetic operation with boolean
constant

ehn(query): set the parameters for system database to be NULL by default when responsing "show databases" command.

fix(query): fix bug in calculating the aggregate function with constant numeric value as input parameter.