Function append_node_to_struct_optional 

pub fn append_node_to_struct_optional(
    struct_builder: &mut StructBuilder,
    vid: Option<Vid>,
    query_ctx: &QueryContext,
)

Append a node to a struct builder, handling the Option<Vid> case.

When vid is Some, delegates to append_node_to_struct. When vid is None, appends a null struct row.