build ios warning: incompatible pointer to integer conversion assigning to 'BOOL' (aka 'signed char') from 'id _Nullable'

` /Users/furun/.pub-cache/hosted/pub.flutter-io.cn/flutter_webview_plugin-0.3.4/ios/Classes/FlutterWebviewPlugin.m:83:22: warning: incompatible pointer to integer conversion assigning to 'BOOL' (aka 'signed char') from 'id _Nullable'
[-Wint-conversion]
_enableAppScheme = call.arguments[@"enableAppScheme"];
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/furun/.pub-cache/hosted/pub.flutter-io.cn/flutter_webview_plugin-0.3.4/ios/Classes/FlutterWebviewPlugin.m:252:24: warning: comparison of distinct pointer types ('NSString *' and 'NSNull * _Nonnull') [-Wcompare-distinct-pointer-types]
if (_invalidUrlRegex != [NSNull null] && urlString != nil) {
~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~
/Users/furun/.pub-cache/hosted/pub.flutter-io.cn/flutter_webview_plugin-0.3.4/ios/Classes/FlutterWebviewPlugin.m:318:94: warning: values of type 'NSInteger' should not be used as format arguments; add an explicit cast to 'long' instead
[-Wformat]
[channel invokeMethod:@"onError" arguments:@{@"code": [NSString stringWithFormat:@"%ld", error.code], @"error": error.localizedDescription}];
~~~ ^~~~~~~~~~
%ld (long)
`

从 ios/Classes/FlutterWebviewPlugin.m 中第 83、252 和 318 行附近的警告开始。构建 iOS 示例或插件以复现编译器诊断，然后验证不兼容转换、指针比较和格式这三个警告已得到解决，且没有引入新的警告。

由索引模型根据 Issue 内容生成。