DatabaseMetaData.getImportedKeys should return real value for PK_NAME column

For consistency with the change made in CONJ-764 and since primary keys are named PRIMARY (see MariaDbDatabaseMetaData.getPrimaryKeys), DatabaseMetaData.getImportedKeys should return PRIMARY (instead of "null") for PK_NAME column.

With the change made for the PK column in exported keys, it is no longer possible to match an exported key from one table with the imported key in the related table.