[FIX] expression: ensure tuple to compare leaf with TRUE/FALSE leaf

When setting a custom filter with as domain
[(0, '=', 1)]

the domain was rejected, because (0, '=', 1) wasn't
considered as a valid leaf, while it is.

This is because the Javascript converts this domain
using list instead of tuple
[(0, '=', 1)] -> [[0, '=', 1]]

And therefore, comparing the "list" leaf
to the TRUE/FALSE leaf tuple failed.

Ensuring "element" as a tuple solves the issue.

opw-640306