This PR was merged into the 2.3 branch.

Discussion
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[Form] Fixed: "required" attribute is not added to <select> tag if no empty value

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8942
| License       | MIT
| Doc PR        | -

Commits
-------

a273e79 [Form] Fixed: "required" attribute is not added to <select> tag if no empty value

… to be omitted from select even though it contains the "multiple" attribute (fabpot)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #10491).

Discussion
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Fixed bug that incorrectly causes the "required" attribute to be omitted from select even though it contains the "multiple" attribute

According to the HTML5 specification the required attribute should be allowed in this case: "A select element with a required attribute and **without** a multiple attribute, and whose size is “1”, must have a child option element."

Related PR: #9030

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

Commits
-------

f9aa676 Fixed bug that incorrectly causes the "required" attribute to be omitted from select even though it contains the "multiple" attribute

This PR was merged into the 2.7 branch.

Discussion
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[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