fix return of default favicon if not found

[will-moore]

Fixes return of default favicon if one is not loaded from domain.

cc @francesw

To test, go to e.g. /mapr/favicon/?u=https%3A%2F%2Flod.humanatlas.io%2Fomap%2F18-lymph-node-cell-dive-ibex%2Fv1.0%2F and you should see a tiny globe icon shown on the page.

[francesw]

looks good on the pilot

[will-moore]

Test above was deployed on pilot idr0159:

sudo /opt/omero/web/venv3/bin/pip uninstall -y omero-mapr
sudo /opt/omero/web/venv3/bin/pip install git+https://github.com/ome/omero-mapr.git@refs/pull/86/head
sudo service omero-web restart

looking at images from idr0158...

[dominikl]

👍 Looks good, thanks Will!