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.test.ts

@@ -27,11 +27,11 @@ describe("LocationSource", () => {
 			expect(items).toEqual([])
 		})
 
-		test("fetchContext returns empty when no location", async () => {
+		test("fetchContext returns null when no location", async () => {
 			const source = new LocationSource()
 
 			const context = await source.fetchContext()
-			expect(context).toEqual({})
+			expect(context).toBeNull()
 		})
 
 		test("fetchContext returns location when available", async () => {