Fix supplier invoice line domain and don't fail for lines without invoice date

issue3803
review4761002
(grafted from 36a03079e9bf0071ac94456e0bc51045b70819dc)

--HG--
branch : 2.8
extra : source : 36a03079e9bf0071ac94456e0bc51045b70819dc
Prepare release 2.8.1

--HG--
branch : 2.8
Added tag 2.8.1 for changeset 2f7c42974ee7

--HG--
branch : 2.8
Increase version number

--HG--
branch : 2.8