caos: Replace uses of bool(Concept) with explicit Concept is [not] None
caos: Replace uses of bool(Concept) with explicit Concept is [not] None

Use explicit tests for None value on Concept classes, since
bool(Concept) semantics is expected to be changed in the future to "if
there is at least one entity in a set represented by >Concept>".

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb