sql: make LIKE accept only TEXT arguments

According to ANSI[1] specification, LIKE arguments must be of
string-like type (VARCHAR, CHAR, TEXT etc). If one of arguments is NULL,
then the result of LIKE function is NULL as well. This patch makes LIKE
follow these rules.

ANSI 2013 Part 2: Foundation; Chapter 8.5 <like predicate>

Closes #3954