Fix using above/below filling option with discontinuous lines

[charlesmass2]

Fixes #10023

Added a test in the boundary folder

[kurkle]

What does spanGaps: true do in this case? And is there a reason it's not used?

[charlesmass2]

@kurkle spanGaps: true would draw a line from the last point of a segment, to the first point of the next segment.
So if one wants to have proper discontinuity in the filling of a line chart, it's not possible with above/below.
Basically, this PR reproduces the behavior of the original commit 18e3bc0 (which was before the 3.0.0 update)

EDIT : Actually, if you'd like to see the difference, you can set spanGaps: true in the two fixtures I added. It might add some clarity 😄

EDIT 2: On our end, we use this to display increasing sets of values over a time period. For instance, increasing values over 1 week with a chart range of 3 weeks. Which would give something like that (once the bug is fixed):

Same config before this bugfix (on master)

Same config with `spanGaps: true`

[kurkle]

Sorry for taking a while before finding the time to properly review.
I had some doubts about having the discontinuity at different spots for dataset and the target, but that seems to work as expected too. Clip is not "grounded" on those spots, but its handled by _fill, so there is no problem.

Good job! 👍

[kurkle]

To visualize this change, clip areas colored for above-below-line-null.json

before

after