ensure result_map objects collection is non-empty

ensure result_map objects collection is non-empty

Fixed issue in cursor handling which affected handling of duplicate
:class:`_sql.Column` or similar objcts in the columns clause of
:func:`_sql.select`, both in combination with arbitary :func:`_sql.text()`
clauses in the SELECT list, as well as when attempting to retrieve
:meth:`_engine.Result.mappings` for the object, which would lead to an
internal error.

Fixes: #11306
Change-Id: I418073b2fdba86b2121b6d00eaa40b1805b69bb8