Add support for the default_page_template_title filter in page-attr…

…ibutes meta-box. (#6948)

* Add support for default_page_template_title filter

* Allow an empty array if there are no templates

* Avoid defining a new function

It's less new code to simply define this inline

editor/components/page-attributes/template.js

@@ -31,7 +31,7 @@ export function PageTemplate( { availableTemplates, selectedTemplate, instanceId
  onBlur={ onEventUpdate }
  onChange={ onEventUpdate }
  >
- { map( { '': __( 'Default template' ), ...availableTemplates }, ( templateName, templateSlug ) => (
+ { map( availableTemplates, ( templateName, templateSlug ) => (
  <option key={ templateSlug } value={ templateSlug }>{ templateName }</option>
  ) ) }
  </select>

lib/client-assets.php

@@ -1033,9 +1033,17 @@ function gutenberg_editor_scripts_and_styles( $hook ) {
  */
  $allowed_block_types = apply_filters( 'allowed_block_types', true, $post );
 
+ $available_templates = wp_get_theme()->get_page_templates( $post_to_edit['id'] );
+ $available_templates = array_merge(
+ array(
+ '' => apply_filters( 'default_page_template_title', __( 'Default template', 'gutenberg' ), 'rest-api' ),
+ ),
+ $available_templates
+ );
+
  $editor_settings = array(
  'alignWide' => $align_wide || ! empty( $gutenberg_theme_support[0]['wide-images'] ), // Backcompat. Use `align-wide` outside of `gutenberg` array.
- 'availableTemplates' => wp_get_theme()->get_page_templates( get_post( $post_to_edit['id'] ) ),
+ 'availableTemplates' => $available_templates,
  'allowedBlockTypes' => $allowed_block_types,
  'disableCustomColors' => get_theme_support( 'disable-custom-colors' ),
  'disablePostFormats' => ! current_theme_supports( 'post-formats' ),