Access Default User Photo
You want to use the default blank user SVG from the Control Panel on the front-end of the site when there is no photo selected?

If so, it's probably easiest just to copy it from vendor/craftcms/cms/web/assets/cp/dist/images/user.svg to a place in your public HTML folder and reference it there.

Something like:

{% if entry.author.getPhoto() %}
    {{ entry.author.getPhoto().url }}
{% else %}
    {{ url('path/to/user.svg') }}
{% endif %}