Grid spacing shows up as 0, 0 in v1.3alpha
#### Summary:
Grid size is reported as 0, 0 in documents created by Inkscape v 1.2.
"inkscape:grid" element does not have "units" defined. v1.2 can handle that.
#### Steps to reproduce:
- open Inkscape v1.3 dev build
- open provided document
- open grid properties page
#### What happened?
- grid spacing is 0, 0
#### What should have happened?
- grid spacing should be 1, 1
Sample attachments:
<!-- Attach the sample file(s) highlighting the issue, if appropriate. -->
#### Version info
```
Inkscape 1.3-alpha (da9044c17b, 2023-04-17)
GLib version: 2.76.1
GTK version: 3.24.37
glibmm version: 2.66.6
gtkmm version: 3.24.7
libxml2 version: 2.9.13
libxslt version: 1.1.37
Cairo version: 1.16.0
Pango version: 1.50.14
HarfBuzz version: 7.1.0
OS version: macOS 13.3.1 (22E261) arm64
```
issue