Skip dumping unique indexes with expressions (RT#93613)

DBD::Pg 3.0.0 returns expression indexes from ->statistics_info, with
COLUMN_NAME => undef and pg_expression => $expression.  Because
->_table_uniq_info just skipped individual rows without COLUMN_NAME,
an index contains a mix of plain columns and expressions would
generate erroneous ->add_unique_constraint() calls.