Update beck-online.js

1. beck-online.js
Added support for beck-online "Leitsatzkartei" LSK (database of leading records), i.e. scraping of articles where beck-online does not have the article itself but only information about the article

2. juris
Added new translator for the German legal database Juris.
I don't know if I understand your question. 
If there is no match, date will be null.
If you mean what we do if we cannot determine the date: I found cases where there is no date because the source in beck-online is an issue number.
We could in such case use the LSK-year. However, the LSK-year only determines that the article was added to beck-online in that year, not that the article is from that year.