[mlir][ODS] Omit printing default-valued attributes in oilists (#68880)

This makes these match the behaviour of optional attributes (which are
omitted when they are their default value of none). This allows for
concise assembly formats without a custom printer.

An extra print of " " is also removed, this does change any existing
uses of oilists, but if the parameter before the oilist is optional,
that would previously add an extra space.

This #68694 + some fixes for the MLIR Python tests, unfortunately GitHub
does not allow re-opening PRs 😕