test: logical source on a triples map can be omitted if all its term maps are constant

[jduchateau]

In RML-Core:

Note
The logical source on a triples map can be omitted if all its term maps produce values irrespective of a logical source. This is the case if all its term maps are constant-valued expression maps, or have rml:BlankNode as term type and no declared expression.

So I guess this can be extended in RML-CC as gather with constant parameters can be executed without logical source ?

See also:
kg-construct/rml-core#274
kg-construct/rml-fnml#82
#56