Fix creating zero length char with MySQL dialect

Fix creating zero length char with MySQL dialect

Fixed issue where string datatypes such as :class:`.CHAR`,
:class:`.VARCHAR`, :class:`.TEXT`, as well as binary :class:`.BLOB`, could
not be produced with an explicit length of zero, which has special meaning
for MySQL. Pull request courtesy J. Nick Koston.

Fixes: #9544
Closes: #9543
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9543
Pull-request-sha: dc17fc3e93f0ba90881c4efb06016ddf83c7af8b

Change-Id: I96925d45f16887f5dfd68a5d4f9284b3abc46d25