fix: not passing iconPath on linux causing broken dialog

[alexmercerind]

If no iconPath is set from FlutterPlatformAlert.showCustomAlert, the string fallbacks to an empty string in the platform channel call. This causes dialog layout to be broken on Linux.

Equivalent Windows implementation checks for an empty string as opposed to a nullptr. While current Linux implementation was checking for a nullptr.

https://github.com/zonble/flutter_platform_alert/blob/13a2c28e6dc9b7424b3aa1d92f22d29189ef8b3d/windows/flutter_platform_alert_plugin.cpp#L376

Before:

Now:

Equivalent code:

final choice = await FlutterPlatformAlert.showCustomAlert(
  windowTitle: 'Update available: v0.2.7',
  text: 'You can\'t skip this one. Can you? 😏',
  positiveButtonTitle: 'Download now',
  negativeButtonTitle: 'Remind me next time',
  neutralButtonTitle: null,
  options: FlutterPlatformAlertOption(
    preferMessageBoxOnWindows: false,
    additionalWindowTitleOnWindows: 'Update',
    showAsLinksOnWindows: true,
  ),
);

[zonble]

Thanks!