Fix clippy warning unwrap_or_default

[patowen]

warning: use of `or_insert_with` to construct default value
  --> common\src\graph_entities.rs:23:40
   |
23 |         let vec = self.map.entry(node).or_insert_with(Vec::new);
   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `or_default()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_default
   = note: `#[warn(clippy::unwrap_or_default)]` on by default