Fix std::optional value() build error on older macOS SDK
No longer happens on the buildbot, but for users building with an older
Xcode , still need to avoid using value().
error:
In file included from /Users/$/blender-source/build_darwin/source/blender/nodes/geometry/CMakeFiles/bf_nodes_geometry.dir/Unity/unity_13_cxx.cxx:9:
/Users/$/blender-source/blender-git/source/blender/nodes/geometry/nodes/node_geo_set_position.cc:36:32: error: 'value' is unavailable: introduced in macOS 10.14
if (math::is_zero(offset.value())) {
^