[IMP] coerce 'falsy' char sizes to None
[IMP] coerce 'falsy' char sizes to None

Otherwise users using fields.char(size=False) will see their content disappear without warning, not cool
MarkusTeufelberger/openobject-server,splbio/openobject-server,gisce/openobject-server,gisce/openobject-server,MarkusTeufelberger/openobject-server,splbio/openobject-server,splbio/openobject-server,gisce/openobject-server,ovnicraft/openerp-server,ovnicraft/openerp-server,MarkusTeufelberger/openobject-server