[Validator] return empty metadata collection if none do exist

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11604
| License       | MIT
| Doc PR        | 
``getPropertyMetadata()`` was introduced in efe42cbb1f284b992d8de9f136c3b20848bee7f9. I guess ``getMemberMetadatas()`` was kept for BC.