Fix/navigated within document null safety

[doonfrs]

Summary

• Fixed null safety issue in NavigatedWithinDocumentEvent.fromJson that was causing runtime errors
• Added proper null handling for url and navigationType fields when they're null in the JSON response

Problem

On some browsers (possibly an old Chrome version), the application was throwing an error: Type 'Null' is not a subtype of type 'String' in type cast when receiving navigation events with null values for the url or navigationType fields.

Solution

Added null-safe casting (as String?) with fallback to empty string (?? '') for both fields to handle null values gracefully.

Test plan

• Verify that navigation events are handled without throwing exceptions
• Test that the fix works with both null and non-null values for url and navigationType

Error Stack Trace:

type 'Null' is not a subtype of type 'String' in type cast
  #0      new NavigatedWithinDocumentEvent.fromJson (package:puppeteer/protocol/page.dart:1353:32)
  #1      PageApi.onNavigatedWithinDocument.<anonymous closure> (package:puppeteer/protocol/page.dart:167:52)
  #2      _MapStream._handleData (dart:async/stream_pipe.dart:247:31)
  #3      _ForwardingStreamSubscription._handleData (dart:async/stream_pipe.dart:184:13)
  #4      _RootZone.runUnaryGuarded (dart:async/zone.dart:1778:10)
  #5      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:381:11)
  #6      _BufferingStreamSubscription._add (dart:async/stream_impl.dart:312:7)
  #7      _ForwardingStreamSubscription._add (dart:async/stream_pipe.dart:154:11)
  #8      _WhereStream._handleData (dart:async/stream_pipe.dart:229:12)
  #9      _ForwardingStreamSubscription._handleData (dart:async/stream_pipe.dart:184:13)
  #10     _RootZone.runUnaryGuarded (dart:async/zone.dart:1778:10)
  #11     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:381:11)