allow all-label filter in grouping

[s1ck]

Currently, users need to specify explicit labels, e.g. [Person],[gender]. However, it might be useful to group all nodes on specific keys. For that, we need to introduce the asterisk in the label filter array.
This also leads to the situation that a node might not have a specific property key and is thus assigned to a null group. The resulting virtual node will have the grouping key(s) assigned to null in that case.