sqlalchemy: fix mysql primary key creation

* Fix checking of Text type supporting the case of NullType column type.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
127:1: F999 redefinition of unused 'compile_text' from line 108
134:1: F999 redefinition of unused 'compile_binary' from line 115
140:1: F999 redefinition of unused 'compile_largebinary' from line 121