[dbapi] fix connect backwards compat harder
Passing None as group and host should result in
"pyro:///cubicweb.appid", not "pyro://None/None.appid". Followup for
2091d275fe5c. Closes #2882666.
rql('SET X name "BoundaryConstraint" ''WHERE X is CWConstraintType, X name "BoundConstraint"')