sql: make GREATEST/LEAST built-ins accept at least two args

Before a46b5200 SQL implementation featured min()/max() functions
overloading: if one argument was passed, then aggregate version would be
invoked; otherwise - scalar one. We decided to get rid of it and rename
scalar version to LEAST()/GREATEST() correspondingly. However, assertion
inside their implementations has been remained: it verifies that number
of passed arguments is greater than 1. On the other hand, now one can
pass literally any number of arguments to this function, including one
(which results in fired mentioned assertion) and zero (which leads to
NULL dereference in expr.c: these functions are marked with
SQL_FUNC_NEEDCOLL flag, and as a consequence they are assumed to have at
least one argument). Firstly, let's place check that number of passed
arguments more than one. Secondly, let's not assume that functions with
SQL_FUNC_NEEDCOLL must have any arguments.

Closes #4453