The nodes update function geo_node_volume_to_mesh_update would not run
if it is not the very first node in the tree.
If the update function is not run, there are sockets not cleared from
the SOCK_UNAVAIL flag (but this needs to be done -- these get available
depending on the chosen mode).

Havent tracked down why this was actually updating when it was the first
node in the tree, but now make sure we always get an update by specifing
an appropriate RNA update callback for the property.