refactor: make fetchContext required on FeedSource

Sources that cannot provide context now return null
instead of omitting the method. The engine checks the
return value rather than method existence.

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-source-location/src/location-source.ts

@@ -73,11 +73,11 @@ export class LocationSource implements FeedSource {
 		}
 	}
 
-	async fetchContext(): Promise<Partial<Context>> {
+	async fetchContext(): Promise<Partial<Context> | null> {
 		if (this.lastLocation) {
 			return { [LocationKey]: this.lastLocation }
 		}
-		return {}
+		return null
 	}
 
 	async fetchItems(): Promise<[]> {