MySQL has traditionally interpreted an empty value in the ESCAPE
clause (ESCAPE '') as no escape character. But when either of the
first two arguments to LIKE is a string with a multi-byte character
set, the empty value is interpreted as meaning backslash should be
used as escape character.

This patch makes LIKE interpret ESCAPE '' the same way regardless of
the character set, and it's taken as meaning there is no escape
character.

It is done by making Item_func_like::eval_escape_clause() check for an
empty escape sequence and handle it before it gets to the part where
it has different branches for different kinds of character sets.

Additionally, this patch makes eval_escape_clause() raise an error if
the specified escape character cannot be converted to the target
character set. The original behavior was to silently fall back to
using backslash as escape character.

Change-Id: I3f9789f3564664154c2bf720b4f81ac5ff6e4fdd