[1.5.x] Fixed #19197 -- fixed convert_values() for nullable numeric fields
[1.5.x] Fixed #19197 -- fixed convert_values() for nullable numeric fields

Cleaned up the implementation of base convert_values() a little, and
made sure it accepts None as a value for numeric fields.

There are no tests attached. The reason is that not all of the
convert_values() accept None as a value for numeric fields (for example
sqlite3.convert_values()).

The reason the base convert_values() needs to accept None is that this
situation might arise in custom compilers for 3rd party backends. It
is easy to keep the convert_values() working, so lets do that.

Backpatch of 12a96bfa263d514884ef11009913b2f8bb163472

hasadna/django,imtapps/django-imt-fork,ccn-2m/django,hasadna/django,alx-eu/django,imtapps/django-imt-fork,bliti/django-nonrel-1.5,bliti/django-nonrel-1.5,alx-eu/django,ccn-2m/django,alx-eu/django,ccn-2m/django,bliti/django-nonrel-1.5,hasadna/django,imtapps/django-imt-fork,ccn-2m/django,alx-eu/django