Avoid failing assert when Physical Web scanner is inactive

ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.h"
 
-#include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/values.h"
 #import "ios/chrome/common/physical_web/physical_web_scanner.h"
 
 IOSChromePhysicalWebDataSource::IOSChromePhysicalWebDataSource() {}
 
 IOSChromePhysicalWebDataSource::~IOSChromePhysicalWebDataSource() {
   StopDiscovery();
 }
 
@@ skipping 12 matching lines @@
   [scanner_ setNetworkRequestEnabled:network_request_enabled];
   [scanner_ start];
 }
 
 void IOSChromePhysicalWebDataSource::StopDiscovery() {
   [scanner_ stop];
   scanner_.reset();
 }
 
 std::unique_ptr<base::ListValue> IOSChromePhysicalWebDataSource::GetMetadata() {
-  std::unique_ptr<base::ListValue> metadata = [scanner_ metadata];
-  DCHECK(metadata);
-  return metadata;
+  if (!scanner_) {
+    return base::MakeUnique<base::ListValue>();
+  }
+  return [scanner_ metadata];
 }
 
 bool IOSChromePhysicalWebDataSource::HasUnresolvedDiscoveries() {
   return [scanner_ unresolvedBeaconsCount] > 0;
 }