[FIX] allow empty filter keys
[FIX] allow empty filter keys

It is now possible to filter for empty keys. This was prevented by a
safe programming error handling. Empty values are valid for testplan
lists e.g. and therefore it is valid to search for them.

renormalist/Tapper-Reports-Web,renormalist/Tapper-Reports-Web,renormalist/Tapper-Reports-Web