Empty union in spherical coordinate system (and cartesian intersection strategy)

Consider the test case:
```
namespace spherical {
using point = boost::geometry::model::d2::point_xy >;
using linestring = boost::geometry::model::linestring;
using polygon = boost::geometry::model::polygon;
using multipolygon = boost::geometry::model::multi_polygon;
using mpoint = boost::geometry::model::multi_point;
const double earthRadiusMts = 6371000;
}

BOOST_AUTO_TEST_CASE(unionTestCase) {
auto p1wkt = "POLYGON((-118.4333440130425 34.01738213274482,-118.3666773220425 34.01738213274482,-118.366677322 34.0166734220001,-118.366677321 34.0000067500001,-118.3666773209543 33.99962584406412,-118.370368063634 33.9889823854999,-118.373872409729 33.983340078,-118.383343992 33.983340078,-118.400010665 33.983340077,-118.400010663 33.9666734050001,-118.416677336 33.9666734040001,-118.433344009 33.9666734030001,-118.43334401 33.9833400750001,-118.416677338 33.983340076,-118.416677339 34.000006747,-118.433344012 34.000006747,-118.433344013 34.016673418,-118.4333440130425 34.01738213274482))";
auto p2wkt = "POLYGON((-118.4779357910156 33.89957063423745,-118.433344012 34.000006747,-118.416677339 34.000006747,-118.3656294438703 34.00264773892163,-118.3666773209543 33.99962584406414,-118.366677319 33.983340078,-118.4779357910156 33.89957063423745))";
spherical::polygon p1,p2;
boost::geometry::read_wkt(p1wkt, p1);
boost::geometry::read_wkt(p2wkt, p2);

spherical::multipolygon out;
boost::geometry::union_(p1,p2,out,boost::geometry::strategy::intersection::cartesian_segments<>());
boost::geometry::union_(p1,p2,out,boost::geometry::strategy::intersection::cartesian_segments<>());
std::cout << "Cartesian union empty: " << boost::geometry::wkt(out) << std::endl;
}
```
with the output:
```
Cartesian union empty: MULTIPOLYGON()
```
Continuing our work with @vosst and as a workaround to https://github.com/boostorg/geometry/issues/475 we tried this, perhaps exotic approach, of using `boost::geometry::strategy::intersection::cartesian_segments<>()` over geometries defined in a spherical coordinate system. We calculated we can mostly live with the consequences in the interim.

But then this happened.