Excluded formulas file created even if effectively empty

For documents with no long formulas to exclude, an exclusion file is still
created, i.e. `gladtex foo.htex`.

The MWE:

```
<html>
<head>
    <meta charset="utf-8">
    </head>
    <body>

Ach du grüne neune<eq>\tau</eq>
</body>
</html>
```

_Originally posted by @humenda in https://github.com/humenda/GladTeX/issues/24#issuecomment-2991834284_
            