[Validator] Fixed string conversion in constraint violations

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10675
| License       | MIT
| Doc PR        | -
I don't think this value *needs* to be very useful. In most cases, validation messages are displayed to the end user. Displaying something  like `['foo', 1, 'Object(Author)']` to an average end users frightens them because they think something is broken.

Developers can always call `$violation->getInvalidValue()` to analyze the value in detail.