cubicweb #2171313 [hooks/syncschema] allow migration with cubes declaring META RTYPES being not yet in the schema [validation pending]

prioritynormal
typebug
done in3.14.3
load0.100
load left0.000
closed by#0067653190c7 [hooks/syncschema] do not crash if the schema did not learn (yet) some new META rtype (closes #2171313)