[TS] Use strict mode in LGraphGroup (#599)

- Adds `NullGraphError` to reduce boilerplate null check code
- Prefer optional `undefined` to explicit `null`
  - Related: https://github.com/Comfy-Org/litegraph.js/issues/595
- Adds strict types

src/LGraph.ts

@@ -869,7 +869,7 @@ export class LGraph implements LinkNetwork, Serialisable<SerialisableGraph> {
       if (index != -1) {
         this._groups.splice(index, 1)
       }
-      node.graph = null
+      node.graph = undefined
       this._version++
       this.setDirtyCanvas(true, true)
       this.change()