Addons manager - check if icon file exists [skip ci]
Addons manager - check if icon file exists [skip ci]

Currently .isNull is used, to detect icon validity, but such method doesn't take the file path detection into an account. Result being a lot of messages outputted to the Report view and fallback icon not being used.

This PR fixes such icon path related issues.
Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master