[CssSelector] don't raise warnings when exception is thrown

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11947
| License       | MIT
| Doc PR        | 

`array_map()` raises a warning when an exception is thrown inside the
callback (see https://bugs.php.net/bug.php?id=55416). To avoid these
warnings, `selectorToXPath()` is applied inside the loop.