Fix wrong coalesce type inference

[janedbal]

Separated from #506

Coalesce is pretty nasty function:

• It picks one of the types given in args and tries to cast all the others to that one (if that one comes to play due to nullability of others).
• The logic differs platform by platform
  • e.g. COALESCE(1, 'foo') is '1' in MySQL and 1 in SQLite and SQL server
• The casting logic is not documented for MySQL at all.
• SQL server has this documented, but it would be pretty complex to implement it precisely:
  • https://learn.microsoft.com/en-us/sql/t-sql/language-elements/coalesce-transact-sql?view=sql-server-ver16&redirectedfrom=MSDN#comparing[…]and-isnull
    • pt 2: "COALESCE returns the data type of value with the highest precedence."
  • https://learn.microsoft.com/en-us/sql/t-sql/data-types/data-type-precedence-transact-sql?view=sql-server-ver16&redirectedfrom=MSDN
    • the precedence list

Therefore, we cannot use the constant types if we do not know the casting logic as it results in wrong types deduced.

[ondrejmirtes]

Thank you.