bug #17787 [Form] Fix choice placeholder edge cases (Tobion)

This PR was merged into the 2.7 branch.

Discussion
----------

[Form] Fix choice placeholder edge cases

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Fixing several problems with choice placeholder that enhances #9030 for more edge cases:

- A choice with an empty value manually added in the choices array should only be considered a placeholder when it is the first element in the final choice select.
This is part of the HTML spec and how browsers also behave. If you select a choice with an empty value that is not the first option, it will still pass the "required" check
and thus submit the empty value. So it's not a placeholder.
If in the example below you move the empty option to the first place, the browsers will error on submit that you
must select a value. So only then it is a placeholder to show as initial value.

```html
<select id="form_timezone" name="form[timezone]" required="required">
    <option value="Africa/Abidjan">Abidjan</option>
    <option value="">Empty</option>
</select>
```

Also the validator https://validator.w3.org/nu/ will mark the above code as error:
> The first child option element of a select element with a required attribute, and without a multiple attribute, and without a size attribute whose value is greater than 1, must have either an empty value attribute, or must have no text content. Consider either adding a placeholder option label, or adding a size attribute with a value equal to the number of option elements.

This is fixed by replacing`0 !== count($choiceList->getChoicesForValues(array('')))` with `$view->vars['placeholder_in_choices'] = $choiceListView->hasPlaceholder()`.
Which means, the required attribute is removed automatically because the select form element is required implicitly anyway due to the nature of the choice UI.

- As the above quote mentions, the `size` attribute also has impact. Namely for a select with size > 1 it can be possible to have a required attribute even without placeholder.
This is because when the size > 1, there is no default choice selected (similar to select with "multiple").

- A placeholder for required radio buttons or a select with size > 1 does not make sense as it would just be fake data that can be submitted (similar to the ignored placeholder for multi-select and checkboxes).

Commits
-------

0efbc30 [Form] fix edge cases with choice placeholder

Author: fabpot

Resources/views/Form/choice_widget_collapsed.html.php

@@ -1,5 +1,5 @@
 <select
-    <?php if ($required && null === $placeholder && $placeholder_in_choices === false && $multiple === false):
+    <?php if ($required && null === $placeholder && $placeholder_in_choices === false && $multiple === false && (!isset($attr['size']) || $attr['size'] <= 1)):
         $required = false;
     endif; ?>
     <?php echo $view['form']->block($form, 'widget_attributes', array(