Double index creation for alterTable

In JDBCStartCommand::hasIndex() the variable idxAscDesc may be null, since null is a valid result value for ASC_OR_DESC in DatabaseMetaData.

This leads to returning false for hasIndex() and therefore creating an index on the column - which already may have one (after first deployment).

Also the implementation currently looks at ascending columns.

If someone shares their ideas on this matter I can cook up a patch.