Error when inserting key for empty track

### Tested versions

4.5 beta4

### System information

W10

### Issue description

https://github.com/user-attachments/assets/0c29433d-4b5c-42fa-ac39-de1b24360c6e

### Steps to reproduce

1. Add AnimationPlayer and animation
2. Create a new property track (tested with `flip_h` of Sprite2D)
3. Insert key
4. Error
```
ERROR: AnimationMixer: 'new_animation', Value Track: 'Sprite2D:flip_h' must have at least one key to cache for blending.
```

### Minimal reproduction project (MRP)

N/A

Reproduce the issue using the listed AnimationPlayer, property-track, and Sprite2D flip_h steps, then search the exact AnimationMixer error to locate its source. Trace why an empty track reaches blending after key insertion; done means inserting the key no longer produces the error and the track can be used normally.

Written by the indexing model from the issue text.