Curve2D get_closest_offset always returns zero if first two points in curve are the same

### Tested versions

- Discovered in v4.3.rc1.official [e343dbbcc]
- Reproduced in v4.2.stable.official [46dc27791]

### System information

Godot v4.3.rc1 - Windows 10.0.22631 - Vulkan (Mobile) - dedicated NVIDIA GeForce GTX 1060 3GB (NVIDIA; 31.0.15.5176) - AMD Ryzen 5 5600X 6-Core Processor (12 Threads)

### Issue description

If a Curve2D has the same position for the first two points, `get_closest_offset` always returns zero, instead of returning the correct offset. With the same curve, if I remove one of the first duplicate points it then returns the correct offset.

### Steps to reproduce

Example code:
```
var curve := Curve2D.new()
curve.add_point(Vector2(100, 100))
curve.add_point(Vector2(100, 100))
curve.add_point(Vector2(200, 200))
var result = curve.get_closest_offset(Vector2(200, 200))
```

1. Run the code, and confirm result incorrectly returns zero (should return ~141)
2. Comment out the third line, thus making the first two points not the same
3. Run the code again, and confirm the result returns the correct offset.

### Minimal reproduction project (MRP)

[curve2doffsettest.zip](https://github.com/user-attachments/files/16403003/curve2doffsettest.zip)

Start by reproducing the issue with the Curve2D example from the report and compare get_closest_offset with and without the duplicate first point. Trace the Curve2D implementation and its closest-offset tests, then confirm that a curve with duplicate initial points returns the expected offset of about 141 rather than zero.

Written by the indexing model from the issue text.