Godot Export generates build with no main scene selected (no warning or error)

### Tested versions

- Reproducible in v4.3.dev6.mono.official [89850d553]

### System information

Godot v4.3.dev6.mono - macOS 14.5.0 - Vulkan (Forward+) - integrated Apple M2 Pro - Apple M2 Pro (10 Threads)

### Issue description

With Godot 4.3-dev6, you can create a project and have it build successfully without having a scene chosen. You will get an error when the application launches, but you will not get a warning that you did not choose a main scene during the export process.

### Steps to reproduce

1. Create a new project
2. Create a 3D Node Scene
3. Save the scene
4. Install the Export Templates
5. Add the platform (MacOS in this case)
7. Export the Project

### Minimal reproduction project (MRP)

[Archive.zip](https://github.com/godotengine/godot/files/15378464/Archive.zip)

Reproduce the issue using the listed steps with a new 3D project, no selected main scene, and a macOS export. Start by tracing the project export flow and determine where export-time validation occurs; done means an export without a main scene clearly reports the missing configuration before launch.

Written by the indexing model from the issue text.