Fix export DECIMAL precision in MariaDB SQL generation (#980)

Fixes #955

MariaDB's parseType() only checked isSized when deciding whether to
append size/precision to column definitions. DECIMAL has isSized=false
but hasPrecision=true, so precision values like DECIMAL(8,2) were
exported as just DECIMAL.

MySQL's parseType() already checks both isSized and hasPrecision. This
aligns MariaDB's behavior with MySQL.

Co-authored-by: JasonOA888 <JasonOA888@users.noreply.github.com>