The revision dd3f918c
introduced a regression:
it was no longer possible to set an icon in the import module,
and if no icon is provided,
the icon should fall back to the base module icon,
which wasn't the case either.

In this last case, it even leaded to a crash:
```
File "/home/odoo/src/14.0/odoo/odoo/addons/base/models/ir_module.py", line 251, in _get_icon_image
    with tools.file_open(path, 'rb') as image_file:
  File "/home/odoo/src/14.0/odoo/odoo/tools/misc.py", line 176, in file_open
    return _fileopen(name, mode=mode, basedir=base, pathinfo=pathinfo, basename=basename, filter_ext=filter_ext)
  File "/home/odoo/src/14.0/odoo/odoo/tools/misc.py", line 211, in _fileopen
    raise ValueError("Unknown path: %s" % name)
Exception
ValueError: Unknown path: /base/static/description/icon.png
```

opw-3340652

closes odoo/odoo#124471

X-original-commit: 77939849
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>