Mapping Search: Don't match empty from/toScheme field with any concept scheme

[stefandesu]

We just had a bug that an invalid concept scheme ended up in jskos-server which had no notation, and because of this rouge concept scheme, the Mapping Search did not work properly. Any search where either fromScheme or toScheme was left empty did not return any results because the empty scheme notation was matched with that invalid concept scheme.

Cocoda should never match an empty notation field with any concept scheme instead.