SvgSerializer: only draw door swing arcs on the storey where the door is cut

Commit d1217e61c4 made the per-drawing SVG group creation lazy, but the
door arc emission kept calling the group accessor unconditionally. Since
the accessor now creates the group on demand, every door with a
SINGLE_SWING_LEFT/RIGHT operation type got its swing arc written into
every storey of the drawing, not just the storey whose section plane
actually intersects the door. Restore the pre-refactor condition so the
arc is only appended when the door already contributed cut geometry to
that storey's group.

AI-generated commit (see AGENTS.md).

src/serializers/SvgSerializer.cpp

@@ -1968,8 +1968,8 @@ void SvgSerializer::write(const geometry_data& data) {
 			}
 		}
 
-		if (!annotation.IsNull()) {
-			write(*po(), annotation);
+		if (po_ != nullptr && !annotation.IsNull()) {
+			write(*po_, annotation);
 		}
 	}