Disallow Empty time_zone Argument in toStartOf* Functions

Why it matters

This feature prevents users from passing an empty time_zone parameter to the toStartOf* date/time functions. This enforces stricter input validation, reducing errors and ensuring that time zone handling is explicit and correct.

How to use it

Users should ensure that the time_zone argument in toStartOf* functions is always specified with a valid non-empty value. For example:

SELECT toStartOfHour(event_time, 'UTC') FROM events;

Passing an empty string as the time zone will now result in an error.