[url_launcher] Fix PlatformException introduced in nnbd release

[nohli]

Description

Fixes PlatformException NOT_A_WEB_SCHEME

forceSafariVC has previously been null (not true) by default.

Since the description (below) still says that it is false (unset) by default, I think, it is desired to set it to false (keeping it as true by default would need updates to the description as well as the if clause causing the PlatformException:

/// [forceSafariVC] is only used in iOS with iOS version >= 9.0. By default (when unset), the launcher
/// opens web URLs in the Safari View Controller, anything else is opened
/// using the default handler on the platform.

  if ((forceSafariVC == true || forceWebView == true) && !isWebURL) {
    throw PlatformException(
        code: 'NOT_A_WEB_SCHEME',
        message: 'To use webview or safariVC, you need to pass'
            'in a web URL. This $urlString is not a web URL.');
  }

Related Issues

flutter/flutter#71725

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[mvanbeusekom]

Can you add a unit-test which simply tests if the default values are correct? This will prevent similar mistake in the future.

[blasten]

this branch has been merged into master. You might want to reopen this PR, but against master.