Light mode resources used for dark mode in some cases

[Hallot]

Describe the bug
In case background_image/background_image_dark are used, but not color or image, then the light resources for branding and background image are used in dark mode.

The issue appears to come from the following in cli_command.dart:

  if (darkImage == null && darkColor == null) {
    if (image != null) {
      darkImage = image;
    } else {
      darkColor = color;
    }
    brandingDarkImage = brandingImage;
    darkBackgroundImage = backgroundImage;
  }

In case we only set background_image_dark, then both darkImage and darkColor will be null, thus brandingDarkImage and darkBackgroundImage will be replaced by their light version.

I tried to fix it in #350, but it appears the fix was incorrect.

To Reproduce
With the following configuration

flutter_native_splash:
background_image: assets/images/app_splash.png
background_image_dark: assets/images/app_splash_dark.png

Expected behavior
The dark background image is used in dark mode when only background_image/background_image_dark are used.

[jonbhanson]

I will push an update soon that will fix this.