Access to Generic filter profiles from SVO using astroquery.svo_fps when using `get_filter_list`

I am using the sub-package [svo_fps](https://astroquery.readthedocs.io/en/latest/svo_fps/svo_fps.html) from `astroquery` to retrieve filter information for a large sample of filters inside a jupyter notebook. I couldn't find a way to access the filter profile info in the [Generic tab of SVO](http://svo2.cab.inta-csic.es/theory/fps/index.php?mode=browse&gname=Generic&asttype=) when using `get_filter_list`.

I can see the filters when using `SvoFps.get_filter_index`. For example, [Generic Bessel U band](http://svo2.cab.inta-csic.es/theory/fps/index.php?id=Generic/Bessell.U&&mode=browse&gname=Generic&gname2=Bessell#filter) has effective lambda at 3659.88 \AA, and when using ` SvoFps.get_filter_index(3650*u.angstrom, 3660*u.angstrom)` I can see the Generic Bessel filter U listed in the output along with an empty ('') facility and instrument.

However, when I try to access all available Generic filters with `get_filter_list`, I get the following errors:

`SvoFps.get_filter_list(facility='Generic')` which yields `IndexError: No filter found for requested Facilty (and Instrument)`

`SvoFps.get_filter_list(facility='')` yields an empty astropy Table.

Start with the astroquery.svo_fps entry points SvoFps.get_filter_list and SvoFps.get_filter_index, then reproduce the two Generic-facility calls in a Jupyter notebook. Compare the filter-index results with the empty or erroring filter-list results; done means Generic filters, including the Bessell U example, can be retrieved through get_filter_list.

Written by the indexing model from the issue text.