[mlir][ods] Allow empty array ref parameter

This patch "fixes" a longstanding issue where the assembly format for
ArrayRefParameter could not handle an empty list. This is because there
was no way to generically optionally parse the first element of the
array. The only solution was to write a (relatively simple) custom parser.

This patch implements "empty" ArrayRefParameters by using
inverted optional groups and an optional ArrayRefParameter.

Depends on D133816

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D133819