This change

* allows using empty arrays `[]` in expessions,
* adds a cut when the leading `[` was encountered, and
* fixes the interaction between arrays and boolean OR.

IMO the restriction that you couldn't use empty arrays is not needed.
The missing cut made error messages slictly worse if you forget to add
the closing `]`.

Filter expressions must not have white spaces before the pipe `|`. The
white space is used to tell a filter expressions, and `std::ops::Or`
apart.