Improve test fixtures

tests/fixtures/events.fixtures.ts

@@ -2,14 +2,14 @@ import type { SSEvent } from '../../src/types.js';
 
 type EventFixture = {
     raw: string;
-    parsed?: SSEvent[];
+    parsed: SSEvent[];
 };
 
 /** Combines all the raw strings into a single chunk and flattens the parsed arrays into a single array to simulate multi-event chunks. */
 const withCombinedChunk = (fixtures: EventFixture[]): EventFixture => {
     return {
         raw: fixtures.map(({ raw }) => raw).join(''),
-        parsed: fixtures.flatMap(({ parsed }) => parsed ?? []),
+        parsed: fixtures.flatMap(({ parsed }) => parsed),
     };
 };