Return true in the presence of NaN in is_empty_or_negative.

Fixes #443 in which rect intersections don't behave as expected if there is a NaN (is_empty_or_negative returned false causing us to return broken intersection rect).
Beyond intersections, we use `is_empty_or_negative` to differentiate between rects with positive non-zero area and everything else which we treat as empty or invalid-in-the-sense-that-it-doesnt-contribute-to-rendering rectangles. NaN should be part of this cathegory.

for euclid 0.21 I plan to remove `is_empty_or_negative` and simply implement `is_empty` everywhere with the semantics of "every thing that is not a valid non-zero area rectangle" since that's what's useful. It's a breaking unfortunately but there's a number them motivating a 0.21 bump.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1650990