[Translation] Fixed regression: When only one rule is passed to transChoice(), this rule should be used

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

There was one regression introduced by #8789, namely when exactly one message is passed to `transChoice()`. Before, the following code was perfectly valid:

```php
$translator = new IdentityTranslator(new MessageSelector());

echo $translator->transChoice('There are two apples', 2, array('%count%' => 2));
```

This didn't fail, because internally, the locale `null` was passed to `MessageSelector`, which always returned the index `0` in that case.

Now, the user's locale is passed, and if the selector returns anything but `0`, an exception is thrown.

I removed this exception for the case that exactly one standard message (without explicit interval) is passed. In that case, this message is returned (just like from `trans()`). If the message key contains more than one message, or if the message has explicit intervals, the exception will still be thrown. See the tests for clarification.

A use case for why no exception should be thrown is validation:

```php
/**
 * @Assert\Length(min=2, minMessage="Please enter at least two characters")
 */
```