[migration/3.18] recover from bad CWSource.in_synchronization default value
The addition of this attribute in 3.13.8 had default=False, which made
no sense, and was fixed in 3.13.9. However no migration was done so
apps that ever saw the 3.13.8 code could still have '' as defaultval for
this attribute.
rql('SET X name "BoundaryConstraint" '
'WHERE X is CWConstraintType, X name "BoundConstraint"')