[Validator] Fix array notation in the PropertyPath::append()
bug #11117 [Validator] Fix array notation in the PropertyPath::append() (jakzal)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #11117).

Discussion
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[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        | -

Commits
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aa6b08d [Validator] Fix array notation in the PropertyPath::append()