The goal of GEOS operations is to always produce valid output. A polygon
with zero extent is invalid, so the closest valid result is `POLYGON
EMPTY`. Another way to put this is that if a polygon collapses completely
due to rounding, it disappears rather than becoming a point.
The collapsed LinearRing result is in fact also invalid. So probably the
current result is out of spec, and should be changed to become
`LINEARRING EMPTY`. (In fact this should be the behaviour of the latest
GEOS version incorporating OverlayNG).
It might be useful to clarify the intent of the flag to keep collapsed
elements; it is not clear what collapsed elements should be retained,
since their collapse implies that they are no longer toplogically valid.
Decisions here also affect line strings as well. Given an input of
"LINESTRING (0 0, 0.1 0.1)" and gridSize of 1, with this flag set, it
returns "LINESTRING (0 0, 0 0)", which is not valid (according to