Title component's "_none" value saved literally when using a custom label

Problem/Motivation

Name fields allow configurable options for the title component (and similarly generational). Options prefixed with -- are intended to represent an empty selection. The default empty option is -- --, which renders correctly as <option value="_none">--</option> and is stored as an empty title value.

When using a custom empty option such as --Please select a Title (which is provided as example in the field settings UI), the form renders the first option incorrectly:
<option value="Please select a Title" selected="selected">Please select a Title</option>

When submitting a form with a name field like this, the name value can become this:

[
  'title' => 'Please select a Title',
  'given' => 'John',
  'middle' => '',
  'family' => 'Doe',
  'generational' => '',
  'credentials' => '',
]

The name gets then rendered as follows:

Please select a Title John Doe

The bug looks like to be in Name::normalizeSelectOptions(). When --Please select a Title is used as empty option, it receives something like the following:

[
  '_none' => 'Please select a Title',
  'Mr.' => 'Mr.',
  'Mrs.' => 'Mrs.',
]

But the code only explicitly searches for a label that starts with '--'. It won't find it, because NameOptionService::getOptions() already has converted options starting with '--' to using the _none key. And having two options starting with '--' is not allowed: "Title options can only have one blank value assigned to it."

It is not safe to remove the logic in Name::normalizeSelectOptions() however, because the name element can also be used without NameOptionService.

Steps to reproduce

1. Add a Name field to an entity type with the title component enabled as a select list (Drop-down field type).
2. On the field settings form, set "Title options" to include a custom empty option, for example: --Please select a Title
3. Create or edit an entity, leave the title at the default first option, fill in at least one of the other name fields (such as "Family") and save.

The saved value on the name field for the title component is now "Please select a Title".

Proposed resolution

In Name::normalizeSelectOptions(), check if one of the options already uses the "_none" key. If so, use that as the value for $empty_label and make sure that option is not added to the $normalized array.

Remaining tasks

• Review
• Merge

User interface changes

Title options configured to start with '--' would render with value="_none".

API changes

None.

Data model changes

None.

A fix and tests will follow.