Function append_edge_to_struct_optional 

pub fn append_edge_to_struct_optional(
    struct_builder: &mut StructBuilder,
    eid: Option<Eid>,
    src_vid: u64,
    dst_vid: u64,
    batch_type_name: Option<String>,
    query_ctx: &QueryContext,
)

Append an edge to a struct builder, handling the Option<Eid> case.

When eid is Some, resolves the type name from batch_type_name (primary) or L0 visibility (fallback), then delegates to append_edge_to_struct. When eid is None, appends a null struct row.