Setting Slur.positions to 0s can still move slurs

[Nate Whetsell]

Hi there,

Thank you for all your work on LilyPond!

Setting Slur.positions to #'(0 . 0) appears to still move slurs. Here’s an 
example that illustrates the behavior:

```
\version "2.22.0"
{
  \override Beam.auto-knee-gap = #8 % This makes the example clearer but isn't 
needed
  c8( c'') c( c'')
  \override Slur.positions = #'(0 . 0)
  c( c'') c( c'')
}
```

The description at—

https://lilypond.org/doc/v2.22/Documentation/snippets/expressive-marks#expressive-marks-moving-slur-positions-vertically
 
<https://lilypond.org/doc/v2.22/Documentation/snippets/expressive-marks#expressive-marks-moving-slur-positions-vertically>

—says: “Positive values move the slur up, and are appropriate for notes with 
stems down. Negative values move downward slurs further down.” Based on this, 
I’d expect setting Slur.positions to #'(0 . 0) to not move slurs at all. 
Instead, it appears that setting Slur.positions to #'(0 . 0) can move slurs 
quite a bit.

All the best,
Nate