default vertical position of bookmarks is not 0

I want to create a toc for a scanned document, which currently looks kinda like
```
"§24 Partielle Ableitungen" 28
"24.1 Satz von Schwarz" 30
"§25 Differenzierbarkeit" 33
```
Using that, the bookmarks link to some small offset in the pdf (roughly as if 35 was given as offset, i.e.
```
"§24 Partielle Ableitungen" 28 35
"24.1 Satz von Schwarz" 30 35
"§25 Differenzierbarkeit" 33 35
```
) and not directly to the top of the page, which is what I would expect them to link to. No offset should mean
```
"§24 Partielle Ableitungen" 28 0
"24.1 Satz von Schwarz" 30 0
"§25 Differenzierbarkeit" 33 0
```

I use version 1.3.4.

No source file or test is named in the issue. Reproduce with the shown bookmark input on version 1.3.4, trace the CLI handling of an omitted vertical offset, and confirm that generated bookmarks use 0 (the top of the page) while explicit offsets remain unchanged.

Written by the indexing model from the issue text.