disable null values for seo metata

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | - 
| License       | MIT
| Doc PR        | not related

As i don't know if is possible yet i implemented a secure condition, that no null values for `SeoMetadata` got into the persistence. (Problem: `SeoMetadata` is "serialized" to an array before update/persist -> array mapped as property with assoc)