make contextId optional in trace file

packages/playwright/src/worker/testTracing.ts

@@ -58,7 +58,6 @@ export class TestTracing {
       wallTime: Date.now(),
       monotonicTime: monotonicTime(),
       sdkLanguage: 'javascript',
-      contextId: '',
     };
     this._appendTraceEvent(this._contextCreatedEvent);
   }

packages/trace-viewer/src/sw/traceModernizer.ts

@@ -92,7 +92,7 @@ export class TraceModernizer {
         contextEntry.sdkLanguage = event.sdkLanguage;
         contextEntry.options = event.options;
         contextEntry.testIdAttributeName = event.testIdAttributeName;
-        contextEntry.contextId = event.contextId;
+        contextEntry.contextId = event.contextId ?? '';
         break;
       }
       case 'screencast-frame': {

packages/trace/src/trace.ts

@@ -44,7 +44,7 @@ export type ContextCreatedTraceEvent = {
   options: BrowserContextEventOptions,
   sdkLanguage?: Language,
   testIdAttributeName?: string,
-  contextId: string,
+  contextId?: string,
 };
 
 export type ScreencastFrameTraceEvent = {