[Validator] Fix array notation in the PropertyPath::append()

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | related  #11072 #11046 (not fixed yet)
| License       | MIT
| Doc PR        | -