[flutter_tool] fix NPE in log reader for web server device (flutter#46729)

Author: Jonah Williams

packages/flutter_tools/lib/src/web/web_device.dart

@@ -218,7 +218,7 @@ class WebServerDevice extends Device {
 
   @override
   DeviceLogReader getLogReader({ApplicationPackage app}) {
-    return _logReader ??= NoOpDeviceLogReader(app.name);
+    return _logReader ??= NoOpDeviceLogReader(app?.name);
   }
 
   @override

packages/flutter_tools/test/general.shard/web/devices_test.dart

@@ -58,6 +58,7 @@ void main() {
     expect(device.supportsScreenshot, false);
     expect(await device.isLocalEmulator, false);
     expect(device.getLogReader(app: mockWebApplicationPackage), isInstanceOf<NoOpDeviceLogReader>());
+    expect(device.getLogReader(), isInstanceOf<NoOpDeviceLogReader>());
     expect(await device.portForwarder.forward(1), 1);
   });