SQL: fix string functions handling of NULL parameter values

[bpintea]

Some string functions that take an integer parameter - like SUBSTRING or INSERT - exhibit a potentially unexpected behavior when any of the integer parameters is computed from a column containing NULL values: the function simply returns the original string [expression] value, applying no operation.
Example: SELECT SUBSTRING(str_column, int_column_with_nulls, some_lenght) = str_column will return true everyWHERE int_column_with_nulls IS NULL.

This is different from directly providing a NULL literal for any of the parameters, when the function returns NULL (as expected).

Note that other RDBMSes (ex: SQL Server, Postgres, MariaDB) return NULL when any parameter - either literal or attribute - is NULL; we should arguably follow lead or at least document the behavior.

[elasticmachine]

Pinging @elastic/es-ql (:Query Languages/SQL)

[bpintea]

Example: SELECT str, int, substring(str, int, 3) AS sub FROM index:

      str      |      int      |      sub      
---------------+---------------+---------------
FURNITURE      |17             |               
OFFICE SUPPLIES|null           |OFFICE SUPPLIES
OFFICE SUPPLIES|9              |UPP            

or SELECT str, num, substring(str, 1, num) AS sub FROM index:

      str      |     num       |      sub      
---------------+---------------+---------------
FURNITURE      |17             |FURNITURE      
OFFICE SUPPLIES|null           |OFFICE SUPPLIES
OFFICE SUPPLIES|9              |OFFICE SU