Stop including empty items in dialog multilistfields

The meaning of empty items for **multi** list fields for categories is
not clear for dialogs (if user does not want to apply any filter, then
the natural way would be to select nothing).

This also caused issues with category fields, when the action script
uses restrictedTraverse to get the uids corresponding to the category
but a path is empty, like for example in 
https://lab.nexedi.com/nexedi/erp5/blob/d51bb0413a806b3db0c5eb69dec06065b9601322/bt5/erp5_accounting/SkinTemplateItem/portal_skins/erp5_accounting/AccountModule_getTrialBalanceReportSectionList.py#L40-48

which does this:

```python
# optional GAP filter
node_uid = []
gap_uid_list = []
for gap in request.get('gap_list', ()):
  gap_uid_list.append(portal.portal_categories.gap.restrictedTraverse(gap).getUid())
if gap_uid_list:
  node_uid = [x.uid for x in portal.portal_catalog(
                                   portal_type='Account',
                                   default_gap_uid=gap_uid_list)] or -1
```

If an empty item is selected, then `gap_uid_list` will contain an entry for 
`portal.portal_categories.gap.restrictedTraverse('').getUid()` which will be the
uid of the gap base category. Searching with a base category uid nowadays does not
match any document, but before 95e3eaec (CMFCategory: Do not index any Base Category
as a related document., 2016-12-21), it was matching all documents having a relation
from this base category and in the case of this trial balance report it was matching all
accounts.

This was a problem for old instances with accounts created before 95e3eaec, because when
they were first indexed, they had the record in category table, so they were matched, but
once they get re-indexed, they no longer had the record, so the result of this report when
selecting the empty item became different, because accounts were no longer included.

Looking back at this, maybe when updating to get 95e3eaec, we should have ran a migration
to delete all these records (re-indexing every document in the background should be enough)
so that if there's a problem, the problem happens right now and not after a few months
after accounts are modified and re-indexed.

When looking at this from end user level, theses empty items not only cause this problem,
but also does not have a clear behaviour and are not needed, so the changes here are about
removing these empty items.

In accounting reports, there was a multi listfield showing all gap categories, "grouped" by
chart of account - but the name of the chart of account was not displayed. This change to
use a None item, which is rendered as disabled to display the chart of account name, but to
do this we had to fix a bug in Formulator, these disabled items were only working properly
for single item widgets, not multiple items widgets.

See merge request !1572