Add STATE::MISSING to allowed states

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1091 https://github.com/Guite/MostGenerator/issues/362
| Refs tickets  | 
| License       | MIT
| Doc PR        | na