NavigationAgent2D GetCurrentNavigationPath returns empty unless IsNavigationFinished is called directly preceding the call

### Tested versions

-Reproducible in Godot 4.4, using C#

### System information

Windows 11, Godot 4.4

### Issue description

NavigationAgent2D has an issue with side effects. If I set a target position without first calling IsNavigationFinished, GetCurrentNavigationPath will always return an empty path, and the PathChanged signal will never trigger.

### Steps to reproduce

1. Create a NavigationAgent2D with a tilemaplayer with a navigation layer
2. Change NavigationAgent2D target position without calling "IsNavigationFinished()"
3. GetCurrentNavigationPath returns empty, PathChanged signal never triggers

### Minimal reproduction project (MRP)

N/A

Start with the NavigationAgent2D entry points GetCurrentNavigationPath, IsNavigationFinished, the target-position update, and the PathChanged signal. Reproduce the issue in Godot 4.4 using the listed tilemap navigation setup, then trace why the path remains empty without the preceding call; done means the path is returned and PathChanged triggers without calling IsNavigationFinished first.

Written by the indexing model from the issue text.