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>
130:1: F999 redefinition of unused 'compile_text' from line 111
137:1: F999 redefinition of unused 'compile_binary' from line 118
143:1: F999 redefinition of unused 'compile_largebinary' from line 124