cubicweb #3038345 [migration] blindly attempt to add unique constraint without checking participants are there [done]

unique constraint where some attribute/relation are missing/not yet unlined should not be added.

prioritynormal
typebug
done in3.17.4
load0.200
load left0.000
closed by#e9fb9377229e [migration.sync_schema_props_perms] ensure all participants to a unique together constraint are there
patch[migration.sync_schema_props_perms] ensure all participants to a unique together constraint are there [applied]