[TS] Use strict mode in LGraphNode - initial (#598)

- Part of effort to convert LGraphNode to TS strict
- Adds some small runtime changes - no impact expected, but it is
possible
- Runtime changes are in separate commits from compile-time type changes
- Risk of downstream impact is low

src/types/serialisation.ts

@@ -91,7 +91,7 @@ export interface ISerialisedGroup {
   id: number
   title: string
   bounding: number[]
-  color: string
+  color?: string
   font_size: number
   flags?: IGraphGroupFlags
 }