[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/infrastructure/NullGraphError.ts

@@ -0,0 +1,6 @@
+export class NullGraphError extends Error {
+  constructor(message: string = "Attempted to access LGraph reference that was null or undefined.", cause?: Error) {
+    super(message, { cause })
+    this.name = "NullGraphError"
+  }
+}