OpenGL: GPUParticles2D with Color alpha set to 0 are not transparent

### Godot version

4.0.dev (8843d9ad347e5c3be5130153aeecdf48e4fe5a14)

### System information

Windows 10

### Issue description

#72853 fixed #72847
But with the same setup and setting the alpha to 0, particles are still visible.

### Steps to reproduce

Reproduce #72847
Set alpha to 0

### Minimal reproduction project

MRP from #72847