[Console] Fix undefined offset when formatting namespace suggestions

[`preg_grep`](http://www.php.net/manual/en/function.preg-grep.php) returns an array indexed using the keys from the input array. `array_values` is used in the method `find`, but was missing in `findNamespace`.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9614
| License       | MIT
| Doc PR        | n/a

Edit: should be merged to 2.4