fix: casbin migration

[adityathebe]

## fixes: 

failed to initialize rbac: 
error creating rbac enforcer: invalid policy rule size: expected 6, got 5, 
rule: [everyone database.kratos * deny true]

setting v5 as empty string didn't fix this. 
Casbin probably still sees the row as 5 columns, so setting it as "na".

Policies formed from permissions table have a valid UUID for v5 and mission-control uses uuid.Validate() to filter them out.