Spec: getActivityById returns ActivityProxy | undefined

nitrogen 0.37 maps `| null` to a variant with NullType, which the Swift
implementation (-> HybridActivityProxySpec?) no longer conformed to.
Optional keeps the same JS behaviour (undefined when not found).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: robertherber

packages/react-native-activity-kit/src/specs/ActivityKitModule.nitro.ts

@@ -43,6 +43,6 @@ export interface ActivityKitModule extends HybridObject<{ ios: 'swift' }> {
     state: AnyMap,
     options?: StartActivityOptions,
   ): ActivityProxy
-  getActivityById(activityId: string): ActivityProxy | null | undefined
+  getActivityById(activityId: string): ActivityProxy | undefined
   getAllActivities(): ActivityProxy[]
 }